In fact, there's a bunch of maps which came with OA and have almost all MP entities, except team items (Doubler, Ammo Regen, Guard, Scout) and new holdables. (Invulnerability & Kamikaze) The only thing is that they can't be seen in the Skirmish menu because of not being mentioned in arenas.txt, except cosmo's ctf_gate1.
You can find and download some maps created for the original Team Arena missionpack at these places. They can work with OA missionpack also, but be aware that most of these maps requires textures which can be only found at the original Q3(T)A:
http://mpmaps.planetquake.gamespy.com/http://lvlworld.com/P.S.: Don't touch mppak0.pk3 nor unzip it in the directory.
P.S.2: AFAIK = As Far As I Know.
You should go to Create Server, instead to New Game. Also, the Custom Game menu item, which should work in the same way OA's Mods menu item work, doesn't work.
