I turned my oven down to around gas mark 3 last night while I finished the rest of dinner. After taking the food out of it, I accidentally left it on for about 5 hours overnight. Our CO meter didn't go off, and I've turned the oven off now. Besides having switched the oven off (if it wasn't obvious) is there anything I need to do to make the situation safe or prevent oven damage? 
No, ovens are designed to function for long periods if necessary, for example in overnight cooking. You might have some burned-on dirt that is harder to clean than usual. I've done this before and been spooked about having a (mild) fire risk due to my own negligence; to reduce the likelihood of it happening again I try to incorporate checking the oven/hobs are off into my routine as I take the food to the table. 