Change order status just after payment in WooCommerce [duplicate]

Update 2 - 2019: Use WooCommerce: Auto complete paid orders (updated thread)

So the right hook to use is woocommerce_payment_complete_order_status filter returning complete

Update 1: Compatibility with WooCommerce version 3+

I have changed the answer

Based on: WooCommerce - Auto Complete paid virtual Orders (depending on Payment methods), you will be able to handle also all payment methods in conditionals:

// => not a filter (an action hook)add_action( 'woocommerce_thankyou', 'custom_woocommerce_auto_complete_paid_order', 10, 1 );function custom_woocommerce_auto_complete_paid_order( $order_id ) {    if ( ! $order_id )        return;    $order = new WC_Order( $order_id );    // No updated status for orders delivered with Bank wire, Cash on delivery and Cheque payment methods.    if ( get_post_meta($order_id, '_payment_method', true) == 'bacs' || get_post_meta($order_id, '_payment_method', true) == 'cod' || get_post_meta($order_id, '_payment_method', true) == 'cheque' ) {        return;     }    // "completed" updated status for paid "processing" Orders (with all others payment methods)    elseif ( $order->has_status( 'processing' ) ) {        $order->update_status( 'completed' );    }    else {        return;    }}

The function woocommerce_thankyou is an action. You're required to use add_action function to hook into it. I would recommend changing the priority to 20 so that other plugins/code changes may be applied before update_order_status.

add_action( 'woocommerce_thankyou', 'update_order_status', 20);