Both OMC and MerCruiser used the 4 cylinder GM engines during that time, and thru today for MerCruiser. It would only be considered a MerCruiser if the outdrive and all externals are MerCruiser, vice-versa for OMC. Mercs were black, OMC's were white outdrive and blue engines. And of course each had their own badges that say OMC or Mercruiser.
Having typed all that, the engines are interchangeable regarding block and heads. Each brand would have their own couplers and intake and exhasut manifolding, fuel systems, cooling water flow, etc. Sometimes in an OMC rigged boat a Merc block could have been swapped in to replace a bad block, something like that. But the rigging is still all OMC or Merc, and that is the determining factor, i.e. what outdrive the block is coupled to.
