This question comes up a few times and you'll get lots of different answers. Whilst Pheasant is right in the sense that the most important aspect of your AIB is your desire to join the RN, there's no point even starting if deep down you're not doing the job you truely want to do.
I have plenty of friends who are O. They love the job and they're damn good at it. However for me it was always very straight forward. I wanted to be a pilot and, whilst taking nothing away from the skill required to be an observer, it was never something I wanted to do. So you need to ask yourself what it is you really want to be. If you're happy being an observer then good luck to you, you'll have a blast. But if deep down its pilot that you want I would echo orca and say stick to your guns.