So, you could get an OCC frame and graft it on to the beach cruiser - but
I think this could be very cool if the beach cruiser maintained a (relatively) stock appearance in its rear triangle material - round tubing, stock dropouts, etc.
there are a few ways to approach this in my mind, but some are more straight forward than others, some require finding/buying parts ($$=UGH to me), some require raw material, and they all require tool, ability and heavy mods.
I am assuming you are using a 26inch cruiser frame - the occ wheel is a 20 inch. Any mod to accommodate the wheel will lower the bike
the biggest issue is clearing the wheel- both at the axle and near the seat post. OCC tires are about 4" across, and the rear frame needs to be wider than that everywhere, not just at the axle. and it needs to be centered and not angling off to one side.
also --pedal clearance of this new rear? the bottom bracket of a standard bike may be an issue
i measured several of my bikes rear ends widths for comparison:
26" regular beach cruiser: 4 ish inches
26" mongoose Dolomite fatty: 6 ish inches
occ 20": 6.75 inches
the occ graft is straight forward, but you need an occ donor frame. And the rectangular tubing won't match. (you might/could lower/angle this new rear triangle and get the bike back to normal height, perhaps)
you could do a similar graft with a fat tire frame (26" or 20" really) a 26" graft would keep the frame height normal.
you could cut off the rear frame arms and widen the unit by welding in new material, then re-attaching to the front half of the frame. Perhaps leave enough original lower arm to clear the pedal cranks, and attach the new assembly there?
The coolest ( and most difficult, IMNHO) would be to leave the rear end intact, heat it with a torch and bend it to the new shape - perhaps around a piece of sewer pipe or similar- it looks like there might be enough material to pull this off if you use a 26" frame to accommodate the 20x4" wheel
anyway, just some thoughts - i like modding frames