I completed the x-8 testing area (the one with gabe and the nightstalkers) and I realized you could replay the tests , both basic and advanced. So if you decide to replay the tests, will the enemies respawn for you to kill? That would be cool, since it would make a good way to gain exp.

Yeah they do. I just spent about twenty minutes farming the nightstalker basic test unusual specimen to get the last rank of abominable. Pro tip: There are 11 Nightstalkers in the X-8 basic test.

Also (although I didn't use it for this), it's a good source of Nightstalker Blood for those into survivalist crafting, as well as eggs and tails for selling.

also books, clipboard etc :whistling:

Gah.. replay the tests.. It took me forever to do the nightstalker ones. I died about 30 times. Of course my character was a non combat build. So in that sense, all of OWB was incredibly difficult for me to get through. Found it very unfair that enemies just spawned so freely while I was trying my best to be sneaky. Sneaking just doesn't work if you can't spot the enemy at a distance so you can go around it.

Hmmm interesting to know... no crashes? as I'm sure I got a lot in Mothership Zeta when using a similar exploit.

Also Abominable is the third easiest to get, just got three ranks in it with melee exclusive.
Black mountain the hill south of Primm for centaurs, the three nightstalker caves and outside areas of them, deathclaws in interior caves all respawn in the vanilla game.