1. You don't have to get an OUPV first. You can start right out at the Master license you qualify for.
2. The tests are nearly identical, neither is "harder" than the other.
A couple of tips I tell everyone that is going to get an OUPV:
a. don't do it
b. get a Masters License, it's no harder.
c. Ensure that you get the "Near Coastal" endorsement (no matter which way you go) it only requires you to document offshore time, no extra testing. That endorsement is perhaps the most abused in the charter boat industry, many don't get it and the law says that you must operate in "INLAND Waters" on your ticket, That means you couldn't for example take a charter to the Bahamas or even the "ledge" in many areas of the Eastern US. You can't cross the COLREGS line which is a lot closer to shore in some areas than people think (all charts clearly show the line).
d. take the "Towing Endorsement" test as well. It's only a 20 question (70% required to pass), easy test, That endorsement now allows you to tow for hire. You never know.........
The best source of information is the Merchant Marine Licensing section of the USCG. You can find out any thing you need right here:
http://www.uscg.mil/stcw/ldcr-deckreq.htm
This is also a good source of information regarding the rules and applicabilty between Inland/International rules at the COLREG demarcation line:
http://www.uscg.mil/vtm/navrules/navrules.pdf
Good Luck