OA is a free(as in freedom) recreation of Quake 3 Arena, not of Elite Force and most certainly not of MOHAA or CoD or Counter-Strike (I don't even know how you got the idea that maps from games based on WWII would even fit into a game based on fiction). OA does have imported and modified maps which are also free(as in freedom) and they fit along the style of OA. OA does not have any maps imported or recreated from commercial games.
"Copying stuff from other games, like Q3A, is bad"
You can use maps as long as they are free(as in freedom). It is bad to use maps that are non-free or from commercial games because of legal reasons.
"We're trying to be as close to Q3A as possible" (different thread)
OA does tend to look like Q3A and has the Q3A style, but is not an identical clone as a lot of the content was created(not recreated), and hopefully with time OA will contain a lot of original and unique content and art.
"It's not at all like the map in Q3A because we moved the railgun platform a little bit downward and got rid of trick-jumping. That makes it totally different."
You got a point there. The map is the one in Q3A, but was modified. But that does not mean that people who work on OA put anything they find on the internet into OA.
If you want you can re-create a map, with the following things in mind: the original map must be free(as in freedom, in OA this means GPLv2), the style must fit with OA, and the map must be fun (a 4-wall frag-festing spawn-camping crap-fests are not very desireable).
Oh, and BTW I think you misinterpreted what fromhell said:
OA is not a scavenge-the-non-Free project.
Notice it says non-Free. This means that maps like wrackdm17 are welcome(as they are freely licendes under GNU GPLv2). All the games you mentioned are commercial games(non-free) and importing maps from them would be bad.
As for recreating maps from commercial games that is possible, but you have to create the map from scratch (you cannot use the original maps as those are non-free) and you have to make sure that the map fits into OA style. I can't think of a single map from CoD that would fit well into OA. I know because I've played all the games you listed.