What I did was:
1) take a Nikon dovetail condenser adapter (I have seen two versions of such adapters, or you can take off the lower part of a Nikon dovetail condenser as adapter);
2) glue a dedicated oil darkfield condenser to the adapter concentrically.
Resulting images are pretty good. Darkfield works up to 97x NA 1.25 oil objective stopped down with funnel stop (to NA around 0.8-0.85), or an iris immersion objective with NA set to NA 0.9.
I have also glued a NA 1.4 condenser, and used Lego spacers to make a filter slot right above the dovetail condenser adapter.
Forum member gekko has also done something similar to what I did, with his Optiphot's dovetail condenser mount.
Yes, Olympus BH2 has a very similar condenser mount.
I used that same dovetail adapter to add a dovetail to an earlier Olympus oil darkfield condenser (from BH maybe?). I took 1" PVC pipe and "turned it down" to the two dimensions I needed to couple the condenser to the adapter. Went together slicker than heck, and works perfectly on my BH2. I used a tablesaw to turn down the pipe to make the adapter, not a lathe.