Most Colleges will be open to the public at some point during the day but because each college reserves the right to close depending on college business you will find that many are closed , often at short notice. Oxford University has now issued a small easy to carry laminated guide to college openings and contact numbers. I am not sure whether Cambridge has as well.
In Oxford for the casual visitor a tour by the Blue Badge Guides is always an excellent introduction to Colleges and College life as the guides know a lot of the porters and can often get you into Colleges at times when it is sometimes closed to the Public.
Cambridge also has walking tours by the Blue Badge so it might be a good idea to start at the Tourist Information Centre when the staff will be able to book a tour for you.
Cambridge and Oxford are both very compact anyway so if you want to just visit under your own steam then walking around and looking at notices in the entrance of each college is quite easy anyway.
Walks and guided tours
One of the nicest things to do in both cities is simply to have a good wander, and also look in each College's doorways for signs about concerts, plays etc.. in the evening. Attending evensong is always a great privilege as is candelit concerts in College Chapels. Somewhere like King's College in Cambridge is just magical with the choir singing at dusk...