<p>Although Math 25a lists BC Calculus as a prerequisite, it is primarily a course in Linear Algebra, which doesn’t use calculus. It is a proof-based course, so the OP’s background with the USAMO could be helpful. </p>
<p>Both Math 23 and Math 25 review single-variable calculus from a more theoretical perspective than that of AP Calculus, and one could fill in the gaps of some practical skills by watching the Khan Academy videos online.</p>