Well I use an OC-2, not an OC-3, but I find lots of uses for it. Mostly I use it on its own just because I like the sound, or I'll use it as a basic "synth" sound and apply dirt and filters to it to get different sounds, sometimes I blend it with other effects that don't produce much low end (my Pulse Synth in particular), sometimes I use it to hit low notes that my bass can't reach, etc.
I don't know if the OC-3 sounds as good though. Some say it doesn't.
And yeah the cheesy '80s fretless stuff is fun too:
http://uk.youtube.com/watch?v=YTWt_pKiMl4
