API docs are here: http://developer.onebusaway.org/modules/onebusaway-applicati...

And all of our products, both server and mobile, are here: https://github.com/OneBusAway/

I don't know what language you prefer working with, but you might find this rather thin Rails site I built last year to support a few features helpful: https://github.com/oneBusAway/onebusaway-deep-links