It appears that Megabus on this route is just selling discounted tickets for https://www.theairlineoxford.co.uk. Note that a return ticket seems to be cheaper booking directly.

So once you know you are not actually looking for a Megabus-branded vehicle it should be very easy to find. I believe I've taken this route before and I don't recall any difficulties. Unless you are taking the 2am bus there will be people around to ask.

T2/3 is a bit of a walk from the terminals but just follow the signs. T5 is on the ground level and you need to look for the bus stand numbers.

The bus will turn up at the expected time, but during rush hours going into Oxford can be very congested so the arrival time may be late (though at quiet times you may arrive earlier than scheduled).