From OA 0.8.8 onwards, a new maplist option was added to allow servers to easier setup maplists and have the maps from the maplist be selected randomly. The option is enabled by ticking "Auto change map" in the map select screen (available in Singleplayer->Skirmish and Multiplayer->Create menus) or by setting the cvar g_autonextmap to 1 (default value is 0). If you place a file named maps_dm.cfg in your "baseoa" folder, then the system will read that file and use it for selecting maps for DeathMatch (Free For All) mode.