Last week I found out my round 1 application to the Columbia MBA has been deferred.

I'm freaking out a bit because that's the only school I applied to. :( All my eggs in one basket so they say.

What happens now? How do I convert into an admit?

Basically, they want to wait: either until the round is over, or for one or more additional rounds - just to see who else they get and how you would compare to them as a candidate.

The good news is you were not rejected. The bad news is that it might be a while until you know which way the decision will go.

Have they been in contact with you? If you speak with them, maybe ask them if there are any concerns with your application. Suggest, for example, retaking the GMAT (assuming you think you could substantially improve your score.)

Good luck!

How long can we expect to wait?

If previous intakes are reliable indicators, you might need to hold tight for a while. In the past, I've known applicants who have been on the Columbia waitlist until January, or even March, as the final deadline approaches. Fortunately, the school tends to be good at communication and keeping applicants updated.