I have a work colleague that works 09:00 to 15:00 but standard business hours are until 17:00.

Is it possible for Out of Office to automatically switch on at 15:00 and turn off at 09:00 Mon-Fri?

I can't see how. I think you are restricted to one time period - so it can just turn on and off, not turn on and off multiple times.

I'd be interested to find an answer as I work part time and it'd be useful if I could set my OOO to turn on every Thursday evening and turn off every Tuesday morning rather than having to reset it every week :-)