Might not be relevant in today's aircrew recruiting climate, but when I did OASC in 1989 I applied for pilot only. Didn't want to end up flying 'in the boot', so told the board I had no interest in being a nav. Bit of a gamble, but it worked out and was given a place at Cranditz for pilot (direct entry BTW).
Factors which tipped the balance may have included being a bit older (21), having done some mil-sponsored assessed flying (30hr RAF flying scholarship) and the numbers going through flying training being slightly higher than at the moment....
Good luck anyway! Be yourself, speak-up and chill out.
ST