nwa.com is a very confusing website, because they actually use two separate systems (the www.nwa.com group of servers, and the res.nwa.com system). www is used for looking up your Worldperks account info, your current reservations, promotions, etc. The res system is used for (you guessed it) reservations.

To set up your Worldperks profile so you can book with a different FF#:

1. Go to http://res.nwa.com/App/ViewMyStuff and sign in with your Worldperks info. (If you don't have a Worldperks number, you should probably sign up for one so you can save your prefs on res.nwa.com--I have a OP# for the same reason on continental.com)
2. Click on "preferences" on the left side of the screen.
3. Under "flight preferences," add you OnePass number (you can also put in other prefs while you're at it).
4. Click "save preferences" at the bottom.
5. Note that you can also set up profiles for "Other Travelers" so if you frequently book travel for family members or friends, you can save all their prefs in the same way under YOUR Worldperks account.

Now, when you book tickets at nwa.com, you can sign in with your WP# and simply select your OP# when you go to check out with your tix. Easy as pie.

Note: another quirk with the www/res distinction is that nwa.com displays seat maps differently on the two systems. When you are in the process of booking your ticket, the seat maps display on the res system and will NOT allow you to choose exit rows even if you're elite. After you book and get a confirmation number, however, you can simply go back to the www.nwa.com home page and look up your reservation there. www DOES allow you to select exit rows as long as you have an eligible WP or OP (or, I assume, other partner elite) FF# in the record.