diff --git a/History.md b/History.md
index 94e72bc609..8067a8e12e 100644
--- a/History.md
+++ b/History.md
@@ -39,6 +39,7 @@
   * Fix `out_of_band` hook never executed if the number of worker threads is > 1 (#2177)
   * Fix ThreadPool#shutdown timeout accuracy (#2221)
   * Fix `UserFileDefaultOptions#fetch` to properly use `default` (#2233)
+  * Improvements to `out_of_band` hook (#2234)
   * Prefer the rackup file specified by the CLI (#2225)
 
 * Refactor
diff --git a/lib/puma/server.rb b/lib/puma/server.rb
index b5269f0ddd..a467e23715 100644
--- a/lib/puma/server.rb
+++ b/lib/puma/server.rb
@@ -224,8 +224,11 @@ def run(background=true)
             @reactor.add client
           end
         end
+
+        process_now
       end
 
+      @thread_pool.out_of_band_hook = @options[:out_of_band]
       @thread_pool.clean_thread_locals = @options[:clean_thread_locals]
 
       if @queue_requests
@@ -279,6 +282,7 @@ def handle_servers
                 break if handle_check
               else
                 begin
+                  pool.wait_until_not_full
                   if io = sock.accept_nonblock
                     client = Client.new io, @binder.env(sock)
                     if remote_addr_value
@@ -288,7 +292,6 @@ def handle_servers
                     end
 
                     pool << client
-                    pool.wait_until_not_full
                   end
                 rescue SystemCallError
                   # nothing
@@ -436,12 +439,6 @@ def process_client(client, buffer)
         rescue StandardError => e
           @events.unknown_error self, e, "Client"
         end
-
-        if @options[:out_of_band]
-          @thread_pool.with_mutex do
-            @options[:out_of_band].each(&:call) if @thread_pool.busy_threads == 1
-          end
-        end
       end
     end
 
diff --git a/lib/puma/thread_pool.rb b/lib/puma/thread_pool.rb
index d6a27c41de..4dcf2f08ab 100644
--- a/lib/puma/thread_pool.rb
+++ b/lib/puma/thread_pool.rb
@@ -47,6 +47,7 @@ def initialize(min, max, *extra, &block)
       @shutdown = false
 
       @trim_requested = 0
+      @out_of_band_pending = false
 
       @workers = []
 
@@ -65,6 +66,7 @@ def initialize(min, max, *extra, &block)
 
     attr_reader :spawned, :trim_requested, :waiting
     attr_accessor :clean_thread_locals
+    attr_accessor :out_of_band_hook
 
     def self.clean_thread_locals
       Thread.current.keys.each do |key| # rubocop: disable Performance/HashEachMethods
@@ -116,6 +118,9 @@ def spawn_thread
               end
 
               @waiting += 1
+              if @out_of_band_pending && trigger_out_of_band_hook
+                @out_of_band_pending = false
+              end
               not_full.signal
               not_empty.wait mutex
               @waiting -= 1
@@ -129,7 +134,7 @@ def spawn_thread
           end
 
           begin
-            block.call(work, *extra)
+            @out_of_band_pending = true if block.call(work, *extra)
           rescue Exception => e
             STDERR.puts "Error reached top of thread-pool: #{e.message} (#{e.class})"
           end
@@ -143,6 +148,21 @@ def spawn_thread
 
     private :spawn_thread
 
+    def trigger_out_of_band_hook
+      return false unless out_of_band_hook && out_of_band_hook.any?
+
+      # we execute on idle hook when all threads are free
+      return false unless @spawned == @waiting
+
+      out_of_band_hook.each(&:call)
+      true
+    rescue Exception => e
+      STDERR.puts "Exception calling out_of_band_hook: #{e.message} (#{e.class})"
+      true
+    end
+
+    private :trigger_out_of_band_hook
+
     def with_mutex(&block)
       @mutex.owned? ?
         yield :
diff --git a/test/test_out_of_band_server.rb b/test/test_out_of_band_server.rb
new file mode 100644
index 0000000000..d81be8985e
--- /dev/null
+++ b/test/test_out_of_band_server.rb
@@ -0,0 +1,159 @@
+require_relative "helper"
+require "puma/events"
+
+class TestOutOfBandServer < Minitest::Test
+  parallelize_me!
+
+  def setup
+    @ios = []
+    @server = nil
+    @oob_finished = ConditionVariable.new
+    @app_finished = ConditionVariable.new
+  end
+
+  def teardown
+    @oob_finished.broadcast
+    @app_finished.broadcast
+    @server.stop(true) if @server
+    @ios.each {|i| i.close unless i.closed?}
+  end
+
+  def new_connection
+    TCPSocket.new('127.0.0.1', @server.connected_ports[0]).tap {|s| @ios << s}
+  rescue IOError
+    Thread.current.purge_interrupt_queue if Thread.current.respond_to? :purge_interrupt_queue
+    retry
+  end
+
+  def send_http(req)
+    new_connection << req
+  end
+
+  def send_http_and_read(req)
+    send_http(req).read
+  end
+
+  def oob_server(**options)
+    @request_count = 0
+    @oob_count = 0
+    in_oob = Mutex.new
+    @mutex = Mutex.new
+    oob_wait = options.delete(:oob_wait)
+    oob = -> do
+      in_oob.synchronize do
+        @mutex.synchronize do
+          @oob_count += 1
+          @oob_finished.signal
+          @oob_finished.wait(@mutex, 1) if oob_wait
+        end
+      end
+    end
+    app_wait = options.delete(:app_wait)
+    app = ->(_) do
+      raise 'OOB conflict' if in_oob.locked?
+      @mutex.synchronize do
+        @request_count += 1
+        @app_finished.signal
+        @app_finished.wait(@mutex, 1) if app_wait
+      end
+      [200, {}, [""]]
+    end
+
+    @server = Puma::Server.new app, Puma::Events.strings, out_of_band: [oob], **options
+    @server.min_threads = options[:min_threads] || 1
+    @server.max_threads = options[:max_threads] || 1
+    @server.add_tcp_listener '127.0.0.1', 0
+    @server.run
+  end
+
+  # Sequential requests should trigger out_of_band after every request.
+  def test_sequential
+    n = 100
+    oob_server
+    n.times do
+      @mutex.synchronize do
+        send_http "GET / HTTP/1.0\r\n\r\n"
+        @oob_finished.wait(@mutex, 1)
+      end
+    end
+    assert_equal n, @request_count
+    assert_equal n, @oob_count
+  end
+
+  # Stream of requests on concurrent connections should trigger
+  # out_of_band hooks only once after the final request.
+  def test_stream
+    oob_server app_wait: true, max_threads: 2
+    n = 100
+    Array.new(n) {send_http("GET / HTTP/1.0\r\n\r\n")}
+    Thread.pass until @request_count == n
+    @mutex.synchronize do
+      @app_finished.signal
+      @oob_finished.wait(@mutex, 1)
+    end
+    assert_equal n, @request_count
+    assert_equal 1, @oob_count
+  end
+
+  # New requests should not get processed while OOB is running.
+  def test_request_overlapping_hook
+    oob_server oob_wait: true, max_threads: 2
+
+    # Establish connection for Req2 before OOB
+    req2 = new_connection
+    sleep 0.01
+
+    @mutex.synchronize do
+      send_http "GET / HTTP/1.0\r\n\r\n"
+      @oob_finished.wait(@mutex) # enter OOB
+
+      # Send Req2
+      req2 << "GET / HTTP/1.0\r\n\r\n"
+      # If Req2 is processed now it raises 'OOB Conflict' in the response.
+      sleep 0.01
+
+      @oob_finished.signal # exit OOB
+      # Req2 should be processed now.
+      @oob_finished.wait(@mutex, 1) # enter OOB
+      @oob_finished.signal # exit OOB
+    end
+
+    refute_match(/OOB conflict/, req2.read)
+  end
+
+  # Partial requests should not trigger OOB.
+  def test_partial_request
+    oob_server
+    new_connection.close
+    sleep 0.01
+    assert_equal 0, @oob_count
+  end
+
+  # OOB should be triggered following a completed request
+  # concurrent with other partial requests.
+  def test_partial_concurrent
+    oob_server max_threads: 2
+    @mutex.synchronize do
+      send_http("GET / HTTP/1.0\r\n\r\n")
+      100.times {new_connection.close}
+      @oob_finished.wait(@mutex, 1)
+    end
+    assert_equal 1, @oob_count
+  end
+
+  # OOB should block new connections from being accepted.
+  def test_blocks_new_connection
+    oob_server oob_wait: true, max_threads: 2
+    @mutex.synchronize do
+      send_http("GET / HTTP/1.0\r\n\r\n")
+      @oob_finished.wait(@mutex)
+    end
+    accepted = false
+    io = @server.binder.ios.last
+    io.stub(:accept_nonblock, -> {accepted = true; new_connection}) do
+      new_connection.close
+      sleep 0.01
+    end
+    refute accepted, 'New connection accepted during out of band'
+  end
+end
diff --git a/test/test_puma_server.rb b/test/test_puma_server.rb
index b7fdb3b215..3179579c72 100644
--- a/test/test_puma_server.rb
+++ b/test/test_puma_server.rb
@@ -984,85 +984,4 @@ def test_http10_connection_header_no_queue
     assert_equal ["HTTP/1.0 200 OK", "Content-Length: 0"], header(sock)
     sock.close
   end
-
-  def oob_server(**options)
-    @request_count = 0
-    @oob_count = 0
-    @start = Time.now
-    in_oob = Mutex.new
-    @mutex = Mutex.new
-    @oob_finished = ConditionVariable.new
-    oob_wait = options.delete(:oob_wait)
-    oob = -> do
-      in_oob.synchronize do
-        @mutex.synchronize do
-          @oob_count += 1
-          @oob_finished.signal
-          @oob_finished.wait(@mutex, 1) if oob_wait
-        end
-      end
-    end
-    @server = Puma::Server.new @app, @events, out_of_band: [oob], **options
-    @server.min_threads = 5
-    @server.max_threads = 5
-    server_run app: ->(_) do
-      raise 'OOB conflict' if in_oob.locked?
-      @mutex.synchronize {@request_count += 1}
-      [200, {}, [""]]
-    end
-  end
-
-  # Sequential requests should trigger out_of_band hooks after every request.
-  def test_out_of_band
-    n = 100
-    oob_server queue_requests: false
-    n.times do
-      @mutex.synchronize do
-        send_http "GET / HTTP/1.0\r\n\r\n"
-        @oob_finished.wait(@mutex, 1)
-      end
-    end
-    assert_equal n, @request_count
-    assert_equal n, @oob_count
-  end
-
-  # Streaming requests on parallel connections without delay should trigger
-  # out_of_band hooks only once after the final request.
-  def test_out_of_band_stream
-    n = 100
-    threads = 10
-    oob_server
-    req = "GET / HTTP/1.1\r\n"
-    @mutex.synchronize do
-      Array.new(threads) do
-        Thread.new do
-          send_http "#{req}\r\n" * (n/threads-1) + "#{req}Connection: close\r\n\r\n"
-        end
-      end.each(&:join)
-      @oob_finished.wait(@mutex, 1)
-    end
-    assert_equal n, @request_count
-    assert_equal 1, @oob_count
-  end
-
-  def test_out_of_band_overlapping_requests
-    oob_server oob_wait: true
-    sock = nil
-    sock2 = nil
-    @mutex.synchronize do
-      sock2 = send_http "GET / HTTP/1.0\r\n"
-      sleep 0.01
-      sock = send_http "GET / HTTP/1.0\r\n\r\n"
-      # Request 1 processed
-      @oob_finished.wait(@mutex) # enter oob
-      sock2 << "\r\n"
-      sleep 0.01
-      @oob_finished.signal # exit oob
-      # Request 2 processed
-      @oob_finished.wait(@mutex) # enter oob
-      @oob_finished.signal # exit oob
-    end
-    assert_match(/200/, sock.read)
-    assert_match(/200/, sock2.read)
-  end
 end