There are people who do remember him from Occidental but no purported classmates remember him from Columbia.
Most likely he never attended Columbia at all. There has never been a transcript produced to indicate the contrary. Until such a transcript is produced, his attendance at Columbia is an unlikely possibility.
Some company publishes Alumni Directories for all the major colleges and universities. They publish one for my alma mater every ten years anyway and I know similar directories exist for Columbia. Several years ago I asked a Columbia graduate if Obama's name appeared in the Columbia directory and he said it did. It's just too much for me to believe that Columbia (which must be the source of alumni contact info - I get contacted by the publisher directly.) was somehow decided to play along with some Obama conspiracy before Obama was Obama. I think it's pretty certain that he has a degree from Columbia, though he probably attended classes there only during his senior year. (And maybe not many classes either. I'm pretty sure I've seen remarks from Columbia profs who remember him.)
ML/NJ
Someone recently wrote an article suggesting that he was a "General Studies" student that went part time or something. That he wasn't actually attending the college as a regular student, but rather as someone who was attending classes for "self improvement," which got him in the door where he otherwise would not have been admitted. I find this notion plausible, and it seems to explain the facts of which I am aware.