If you accept qn offer, can you change your mind afterwards?

I am forced to do the Finnish national service next year so I will have a gap year. Hence, I am applying for a deferred entry. However, I didn't apply to Oxbridge and kinda regret it now.

My question is, if I get an offer for a deferred entry now and accept it, can I reapply to Ox/Cam next year and, should I be admitted, go there instead and just dump the Uni that accepted me this year. Or am I oblidged to go to whatever uni I accept an offer from?

You would have to withdraw from the original uni to be able to reapply