Yes, P&O have unfortunately closed the loophole of being able to launder your OBC through the casino. A few years ago we discovered by accident that we could, for example, charge about £80 of our OBC to our casino credit, play the slots for a short while and then get the £70 or so remaining as cash at the desk.
You will find that it won't take you long to burn through it; on a recent trip with our three adult kids and their partners, after a couple of meals in speciality restaurants, wine for the table, and late night beverages in the bars, the old man had spent his £460 OBC within five days, whereas the wily kids still had theirs. 🙄