Auto Populate Custom Field with Complex Value That Increase by One?

I suppose you need somewhere to globally store last order number. Preferable in table wp_options. Method update_option will help you.
Then you need to make a function which will fires everytime when new order appears.

something like that:

add_action( 'save_post', 'setOrderNumber');
function setOrderNumber($post_id) {

  $slug = 'orders'; // slug of your post_type called "Orders"

  if ( $slug != $_POST['post_type'] ) {
        return;
    }
  if ( !current_user_can( 'edit_post', $post_id ) ) {
        return;
    }

  $counter = get_option( 'lastOrderCount' ); // example 0051
  $counter++; // returns 0052
  update_post_meta($post_id, 'purchase_order', 'POCOS'.date('Y').$counter);
  update_option( 'lastOrderCount', $counter ); // set the new number of order. 
}

I didn’t checked this function, but I hope it works.