I've used Pentax SMC-A/M 50mm f/2 lenses for the OtterVIS, but of course others will work as well.
This week I got my hands on two different 50mm Olympus lenses. Here are some thoughts on them:
The very easy to find Zuiko 50mm f/1.8 is not really very suitable. Once taken apart, the lens barrel isn't much smaller than the lens in it's entirety.
The less abundant Olympus PF 50m f/2 is a different story. Once removed from the focusing helicoid, the lens barrel is quite small. And it's almost as if it's made to be taken apart. There are very few screws to remove and the lens groups are held together with holders with bayonetts. One drawback is that there are no screw holes to secure the lens to spectrometer.
The lens is from the 'power focus line' (hence the PF designation) and the demand for them is small since they are virtually unusable as manual focus lenses, so if you can find them they are likely to be much cheaper than their predecessors. Without knowing for sure, I'd say the optics are identical to the venerable Zuiko 50/1.8.