<?php
add_action( 'woocommerce_thankyou', 'custom_woocommerce_auto_complete_order' );
function custom_woocommerce_auto_complete_order( $order_id ) {
global $woocommerce;
if ( !$order_id )
return;
$order = new WC_Order( $order_id );
$order->update_status( 'processing' );
}
?>