I shall await Beagle's response to your last post with eager anticipation.
Re your original query. Its not rocket science is it? A final year student (as your profile suggests) should have the intelligence and common sense to work it all out (unless, of course, you are doing an Arts degree)
The advice to go the day before seems to me to be the best, as you would arrive at OASC fresh and more able to succeed in the tests, rather than stressed out from travelling the same day. Plus it would allow you to spend all the previous day travelling to the middle of nowhere that is Cranwell if needs be.
I wonder how many people I've annoyed with that last bit. Cranwell is lovely, if a bit remote. A bit like Cornwall actually !