If you apply before you've done your A-levels, predicted grades are quite important, but your AS and GCSE grades, interview performance, written work and tests will all be considered as well. If your predicted grades are low but you do well in your AS-levels, your teachers should increase your predicted grades anyway. If you don't do very well at AS and still have low predicted grades, you should probably think about whether applying to Oxbridge is really the right thing to do considering that almost everyone will be predicted AAA+, but if you're confident you can get those grades, there's no harm in applying. Just be aware that the rest of your application will have to be very strong to make up for it.