diff --git a/includes/class-amp-theme-support.php b/includes/class-amp-theme-support.php
index 445fb9d6e9f..0a3ac36cd80 100644
--- a/includes/class-amp-theme-support.php
+++ b/includes/class-amp-theme-support.php
@@ -77,12 +77,12 @@ class AMP_Theme_Support {
 	public static $headers_sent = array();
 
 	/**
-	 * Output buffering level when starting.
+	 * Whether output buffering has started.
 	 *
 	 * @since 0.7
-	 * @var int
+	 * @var bool
 	 */
-	protected static $initial_ob_level = 0;
+	protected static $is_output_buffering = false;
 
 	/**
 	 * Initialize.
@@ -261,11 +261,12 @@ public static function add_hooks() {
 		 * Start output buffering at very low priority for sake of plugins and themes that use template_redirect
 		 * instead of template_include.
 		 */
-		add_action( 'template_redirect', array( __CLASS__, 'start_output_buffering' ), 0 );
+		$priority = defined( 'PHP_INT_MIN' ) ? PHP_INT_MIN : ~PHP_INT_MAX; // phpcs:ignore PHPCompatibility.PHP.NewConstants.php_int_minFound
+		add_action( 'template_redirect', array( __CLASS__, 'start_output_buffering' ), $priority );
 
 		// Add validation hooks *after* output buffering has started for the response.
 		if ( AMP_Validation_Utils::should_validate_response() ) {
-			add_action( 'template_redirect', array( 'AMP_Validation_Utils', 'add_validation_hooks' ), 1 );
+			AMP_Validation_Utils::add_validation_hooks();
 		}
 
 		// Commenting hooks.
@@ -978,6 +979,20 @@ public static function start_output_buffering() {
 		}
 
 		ob_start( array( __CLASS__, 'finish_output_buffering' ) );
+		self::$is_output_buffering = true;
+	}
+
+	/**
+	 * Determine whether output buffering has started.
+	 *
+	 * @since 0.7
+	 * @see AMP_Theme_Support::start_output_buffering()
+	 * @see AMP_Theme_Support::finish_output_buffering()
+	 *
+	 * @return bool Whether output buffering has started.
+	 */
+	public static function is_output_buffering() {
+		return self::$is_output_buffering;
 	}
 
 	/**
@@ -990,6 +1005,7 @@ public static function start_output_buffering() {
 	 * @return string Processed Response.
 	 */
 	public static function finish_output_buffering( $response ) {
+		self::$is_output_buffering = false;
 		return self::prepare_response( $response );
 	}
 
diff --git a/includes/utils/class-amp-validation-utils.php b/includes/utils/class-amp-validation-utils.php
index 57af4b989c4..36c57d6d4ad 100644
--- a/includes/utils/class-amp-validation-utils.php
+++ b/includes/utils/class-amp-validation-utils.php
@@ -943,6 +943,11 @@ public static function get_source( $callback ) {
 	 */
 	public static function can_output_buffer() {
 
+		// Output buffering for validation can only be done while overall output buffering is being done for the response.
+		if ( ! AMP_Theme_Support::is_output_buffering() ) {
+			return false;
+		}
+
 		// Abort when in shutdown since printing we know to be in buffering display handler, and no output allowed anyway.
 		if ( did_action( 'shutdown' ) ) {
 			return false;
@@ -1003,7 +1008,7 @@ public static function wrapped_callback( $callback ) {
 			}
 			$result = call_user_func_array( $function, array_slice( $args, 0, intval( $accepted_args ) ) );
 			if ( $has_buffer_started ) {
-				@ob_end_flush(); // phpcs:ignore Generic.PHP.NoSilencedErrors.Discouraged
+				ob_end_flush();
 			}
 			array_pop( self::$hook_source_stack );
 
diff --git a/tests/test-class-amp-theme-support.php b/tests/test-class-amp-theme-support.php
index d3653a8c00f..5e0ced91953 100644
--- a/tests/test-class-amp-theme-support.php
+++ b/tests/test-class-amp-theme-support.php
@@ -278,7 +278,8 @@ public function test_add_hooks() {
 		$this->assertEquals( 10, has_filter( 'customize_partial_render', array( self::TESTED_CLASS, 'filter_customize_partial_render' ) ) );
 		$this->assertEquals( 10, has_action( 'wp_footer', 'amp_print_analytics' ) );
 		$this->assertEquals( 100, has_filter( 'show_admin_bar', '__return_false' ) );
-		$this->assertEquals( 0, has_action( 'template_redirect', array( self::TESTED_CLASS, 'start_output_buffering' ) ) );
+		$priority = defined( 'PHP_INT_MIN' ) ? PHP_INT_MIN : ~PHP_INT_MAX; // phpcs:ignore PHPCompatibility.PHP.NewConstants.php_int_minFound
+		$this->assertEquals( $priority, has_action( 'template_redirect', array( self::TESTED_CLASS, 'start_output_buffering' ) ) );
 
 		$this->assertEquals( PHP_INT_MAX, has_filter( 'wp_list_comments_args', array( self::TESTED_CLASS, 'set_comments_walker' ) ) );
 		$this->assertEquals( 10, has_filter( 'comment_form_defaults', array( self::TESTED_CLASS, 'filter_comment_form_defaults' ) ) );
diff --git a/tests/test-class-amp-validation-utils.php b/tests/test-class-amp-validation-utils.php
index 6217f7c9323..c4954fe388a 100644
--- a/tests/test-class-amp-validation-utils.php
+++ b/tests/test-class-amp-validation-utils.php
@@ -389,7 +389,7 @@ public function test_wrap_widget_callbacks() {
 		) );
 		$_wp_sidebars_widgets[ $sidebar_id ] = array( $widget_id ); // WPCS: global override ok.
 
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		dynamic_sidebar( $sidebar_id );
 		$output = ob_get_clean();
 
@@ -440,21 +440,21 @@ public function test_callback_wrappers() {
 		$this->assertEquals( 10, has_action( $action_one_argument, array( $this, 'output_notice' ) ) );
 		$this->assertEquals( 10, has_action( $action_two_arguments, array( $this, 'output_message' ) ) );
 
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		do_action( $action_function_callback );
 		$output = ob_get_clean();
 		$this->assertContains( '<div class="notice notice-error">', $output );
 		$this->assertContains( '<!--amp-source-stack {"type":"plugin","name":"amp"', $output );
 		$this->assertContains( '<!--/amp-source-stack {"type":"plugin","name":"amp"', $output );
 
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		do_action( $action_no_argument );
 		$output = ob_get_clean();
 		$this->assertContains( '<div></div>', $output );
 		$this->assertContains( '<!--amp-source-stack {"type":"plugin","name":"amp"', $output );
 		$this->assertContains( '<!--/amp-source-stack {"type":"plugin","name":"amp"', $output );
 
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		do_action( $action_one_argument, $notice );
 		$output = ob_get_clean();
 		$this->assertContains( $notice, $output );
@@ -462,10 +462,10 @@ public function test_callback_wrappers() {
 		$this->assertContains( '<!--amp-source-stack {"type":"plugin","name":"amp"', $output );
 		$this->assertContains( '<!--/amp-source-stack {"type":"plugin","name":"amp"', $output );
 
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		do_action( $action_two_arguments, $notice, get_the_ID() );
 		$output = ob_get_clean();
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		self::output_message( $notice, get_the_ID() );
 		$expected_output = ob_get_clean();
 		$this->assertContains( $expected_output, $output );
@@ -473,21 +473,21 @@ public function test_callback_wrappers() {
 		$this->assertContains( '<!--/amp-source-stack {"type":"plugin","name":"amp"', $output );
 
 		// This action's callback doesn't output any HTML tags, so no HTML should be present.
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		do_action( $action_no_tag_output );
 		$output = ob_get_clean();
 		$this->assertNotContains( '<!--amp-source-stack ', $output );
 		$this->assertNotContains( '<!--/amp-source-stack ', $output );
 
 		// This action's callback comes from core.
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		do_action( $action_core_output );
 		$output = ob_get_clean();
 		$this->assertContains( '<!--amp-source-stack {"type":"core","name":"wp-includes"', $output );
 		$this->assertContains( '<!--/amp-source-stack {"type":"core","name":"wp-includes"', $output );
 
 		// This action's callback doesn't echo any markup, so it shouldn't be wrapped in comments.
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		do_action( $action_no_output );
 		$output = ob_get_clean();
 		$this->assertNotContains( '<!--amp-source-stack ', $output );
@@ -507,7 +507,7 @@ public function test_callback_wrappers() {
 		};
 		add_action( 'outer_action', $handle_outer_action );
 		add_action( 'inner_action', $handle_inner_action );
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		do_action( 'outer_action' );
 		$output = ob_get_clean();
 		$this->assertEquals(
@@ -579,7 +579,7 @@ public function test_wrapped_callback() {
 
 		$wrapped_callback = AMP_Validation_Utils::wrapped_callback( $callback );
 		$this->assertTrue( $wrapped_callback instanceof Closure );
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		call_user_func( $wrapped_callback );
 		$output = ob_get_clean();
 
@@ -600,7 +600,7 @@ public function test_wrapped_callback() {
 
 		$wrapped_callback = AMP_Validation_Utils::wrapped_callback( $callback );
 		$this->assertTrue( $wrapped_callback instanceof Closure );
-		ob_start();
+		AMP_Theme_Support::start_output_buffering();
 		$result = call_user_func( $wrapped_callback );
 		$output = ob_get_clean();
 		$this->assertEquals( 'Closure', get_class( $wrapped_callback ) );