Automatically change the status for an order placed with a Check or BACS gateway

gistfile1.php

<?php

// Automatically update the order status to "completed" for cheque orders
function skyverge_auto_complete_on_hold_order( $order_id ) {

	$order = new WC_Order( $order_id );

	if ( 'on-hold' === $order->status )
		$order->update_status( 'completed' );
}
add_action( 'woocommerce_thankyou', 'skyverge_auto_complete_on_hold_order' );