OMC was the parent company for Johnson and Evinrude. Under the Bombardier umbrella now.
Since you currently have a Mercury motor, I don't think you've got Evinrude wiring and cables. So the new motor's wires and remote won't be plug and play like it would if you were pulling an Evinrude/Johnson.
Where are you getting the 55hp? Get the wiring, cables and remote that go w/ the Evinrude.