To start with, the MMD (Now known as the MMC or Merchant Mariners Credential) is mandatory for all hands aboard U.S.-documented vessels of 100 GRT and up (U.S. Domestic tonnage). I repeat, ALL HANDS: Deckhands, Wipers, Engineers, Cooks, Captains, Mates, Mermaids, Pirates and so on.
Next, an AB-OSV is, very simply, only good on OSV's (Offshore Service Vessels). That's why it says OSV on it. It's no good on a tug or ANY other kind of vessel. Just OSV's. But it's good wherever, not just in the GOM, out to 200 NM and beyond.
Any company that tells you that they will use an AB-OSV or AB-Sail or a Wiper in place of the required AB (Special, Limited or Unlimited, as the case may be) is fly-by-night and should be avoided. Given the Seattle - Alaska route you described it may be that they're documented as commercial fishing industry vessels. If that's the case an AB-Fishing Industry would be sufficient.
The regulations concerning AB's can be found in 46 CFR - Part 12 (Certification Of Seamen) and in 46 CFR - Part 15 (Manning Requirements).