Your flat out wrong ATERP...you need to see how planes are certified...and how instrument approaches are certified...
Simply put, if your looking at the climb gradient of a MAP or departure, and are responsible enough to your passengers, industry, owner of the aircraft, your family, your profession...then you will pull out the Flight Manual and see if you have the single engine performance to do that procedure.
I don't get how the OEI differs from the IAP procedures...if by that you mean the math that the pilot has to accomplish to get to a climb gradient or feet/nm...that's the pilot's job...
Either way...the numbers need to be crunched if you are actually doing your job.