Figure a simple way to solve the issue of hiding out of stock without doing any modification to the opencart source file. Using a trigger that checks the status of the order, if its status changes to Complete(status_id =5) it disables the product by setting the product status to 0

CREATE TRIGGER `trigger_name` AFTER UPDATE ON `oc_order`
FOR EACH ROW if NEW.order_status_id = 5 then
update oc_product
set status = 0
where product_id = (select product_id from oc_order_product where order_id = NEW.order_id)
and quantity = 0;
end if

Execute this sql statement on the table oc_order and change the name tigger_name to your preferred name.