Add prefix before order id

Sounds like you need sequential numbers

https://www.woothemes.com/products/sequential-order-numbers-pro/

https://en-gb.wordpress.org/plugins/woocommerce-sequential-order-numbers/

Always, or that plugin would not be possible. You’ll need to write the filter/function yourself though https://github.com/woothemes/woocommerce/blob/master/includes/abstracts/abstract-wc-order.php#L1044