Well, do you see one in your boat?? <GRIN!>
However, no there wasn't originally a compression post on the 1977 O'DAY 20. The cabin top was apparently arched enough to not need one, presumably there was some form of re-enforcement molded in under the mast step.
The 1973-75 models did have a compression post as part of the main bulkhead under the mast step location. In 1976 O'DAY changed the interior layout of the 20 and eliminated the bulkhead, opening up the interior to have more space. At that time they also changed from the original masthead rig to the taller, Fractional rig.