random won't be possible (afaik), but you can make a map rotation. Put something like this in maps.cfg (in the baseoa directory)
bots_enable 1 // not sure if this is the right variable! could also be bot_enable or bots_enabled
bot_minplayers 5
set m1 "map oasago2; set nextmap m2;"
set m2 "map pul1ctf; set nextmap m3"
set m3 "map ps37ctf; set nextmap m4"
set m4 "map speedyctf; set nextmap m5"
set m5 "map pul1ctf; set nextmap m6";
set m6 "map ctf_gate1; set nextmap m7";
set m7 "map speedyctf; set nextmap m1";
vstr m1;
then when openarena is open put this in the console:
/exec maps.cfgthis can be expanded a lot (could give some examples if you need) but the basic idea should be working