If it's a persuading management thing then I'd always argue that if I left and someone else had to look after the code then it would be easier and more cost-effective with regards to training and hand-over for it to be written in a proper OO way. I'd also always argue that something written well is always easier to extend later on if needed, whereas something that isn't is more likely to cause problems later on. That last argument is always easier to argue if the system hasn't been running without problems for a while.
