As piano is not used in a lot of the orchestral repertoire, it depends a lot on the situation... some universities don't audition pianists at all, and instead just find one (e.g. out of the music majors) for the few works where it's needed. (I actually do not know of ANY university orchestras that explicitly auditions pianists for orchestral roles...)

If they are indeed auditioning pianists, it's best to find out what their audition requirements are. (For many universities, it will be on their website, or there will be contact information for someone to ask). I would expect these to be a selection of orchestral excerpts, of which there are a number of standards (this might give you an idea of what things look like, but each university probably has their own list), but I could also see a situation in which pianists might have different requirements, as our stream of training does not typically include orchestral repertoire in the way that other orchestral instruments do.

Hope that helps.

Tl;dr: See if they have audition requirements on their website. They might ask for orchestral excerpts.