When HPOS is enabled, wc_get_order() should still utilize global post and order objects (if needed).

plugins/woocommerce/changelog/fix-35561-wc-get-order

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fix
+
+Ensure wc_get_order() works without arguments when HPOS is enabled.

plugins/woocommerce/includes/class-wc-order-factory.php

@@ -10,6 +10,8 @@
 
 defined( 'ABSPATH' ) || exit;
 
+use Automattic\WooCommerce\Utilities\OrderUtil;
+
 /**
  * Order factory class
  */
@@ -165,10 +167,8 @@ class WC_Order_Factory {
 	 * @return int|bool false on failure
 	 */
 	public static function get_order_id( $order ) {
-		global $post;
-
-		if ( false === $order && is_a( $post, 'WP_Post' ) && 'shop_order' === get_post_type( $post ) ) {
-			return absint( $post->ID );
+		if ( false === $order ) {
+			return self::get_global_order_id();
 		} elseif ( is_numeric( $order ) ) {
 			return $order;
 		} elseif ( $order instanceof WC_Abstract_Order ) {
@@ -180,6 +180,33 @@ class WC_Order_Factory {
 		}
 	}
 
+	/**
+	 * Try to determine the current order ID based on available global state.
+	 *
+	 * @return false|int
+	 */
+	private static function get_global_order_id() {
+		global $post;
+		global $theorder;
+
+		// Initialize the global $theorder object if necessary.
+		if ( ! isset( $theorder ) || ! $theorder instanceof WC_Abstract_Order ) {
+			if ( ! isset( $post ) || 'shop_order' !== $post->post_type ) {
+				return false;
+			} else {
+				OrderUtil::init_theorder_object( $post );
+			}
+		}
+
+		if ( $theorder instanceof WC_Order ) {
+			return $theorder->get_id();
+		} elseif ( is_a( $post, 'WP_Post' ) && 'shop_order' === get_post_type( $post ) ) {
+			return absint( $post->ID );
+		} else {
+			return false;
+		}
+	}
+
 	/**
 	 * Gets the class name bunch of order instances should have based on their IDs.
 	 *

plugins/woocommerce/tests/legacy/unit-tests/order/class-wc-tests-order-functions.php

@@ -162,6 +162,11 @@ class WC_Tests_Order_Functions extends WC_Unit_Test_Case {
 	 * @group test
 	 */
 	public function test_wc_get_order() {
+		global $post;
+		global $theorder;
+
+		$original_post     = $post;
+		$original_theorder = $theorder;
 
 		$order = WC_Helper_Order::create_order();
 
@@ -178,11 +183,29 @@ class WC_Tests_Order_Functions extends WC_Unit_Test_Case {
 		$post = $this->factory->post->create_and_get( array( 'post_type' => 'post' ) );
 		$this->assertFalse( wc_get_order( $post->ID ) );
 
+		// Assert the return when $the_order args is a random (incorrect) id.
+		$this->assertFalse( wc_get_order( 123456 ) );
+
 		// Assert the return when $the_order args is false.
 		$this->assertFalse( wc_get_order( false ) );
 
-		// Assert the return when $the_order args is a random (incorrect) id.
-		$this->assertFalse( wc_get_order( 123456 ) );
+		$post = get_post( $order->get_id() );
+		$this->assertInstanceOf(
+			'WC_Order',
+			wc_get_order(),
+			'If no order ID is specified, wc_get_order() will use the global $post object to try and determine the current order.'
+		);
+
+		unset( $post );
+		$theorder = $order;
+		$this->assertInstanceOf(
+			'WC_Order',
+			wc_get_order(),
+			'If no order ID is specified, wc_get_order() will use the global $theorder object to try and determine the current order.'
+		);
+
+		$post     = $original_post;
+		$theorder = $original_theorder;
 	}
 
 	/**