i know this thread is kinda old but ill share what i suggest and my reasoning behind them.
/cg_lagometer - turns on the lagometer if its not already on
/cg_showfps - shows your fps onscreen, useful for checking com_maxfps
/cg_drawtimer - shows a timer that tells you how long the map has been running
/rate 25000 - the single most important setting IMO, a good 75% of high ping people you see on OA have this set to the default of 3000, the rest are just REALLY far away from the server.
/snaps 20 30 or 40 depending on the server (do a /sv_fps to see what to use)
/cl_maxpackets 30 - only defrag and cpma mods need 125 here, otherwise you are artificially raising your ping
/cl_packetdup 1 - if you get lots of red on the lagometer try setting this to 2
/cg_smoothClients 0 - only change to 1 if you are going to record demos
/cg_truelightning 1 - straightens out the lightning trail significantly, making it easier to aim
/cg_fov 100 - i leave mine at 90, 110 gives a strange fisheye effect
/com_hunkmegs 128 - only change if you have more than 512MB of ram or so
/com_zonemegs 32 - helps for larger maps it seems
/com_soundmegs 16 - probably helps if your sound stutters from time to time
/com_maxfps 125 - use 76 or 43 if your computer cannot handle 125, they are almost as good physics-wise
these settings are based on a few quake 3 tweak/settings sites i have found and personal experience. here are a few links i suggest anyone who wants to change settings in open arena should read:
http://ucguides.savagehelp.com/Quake3/connection.html - more detailed connection settings guide
http://ucguides.savagehelp.com/Quake3/FAQFPSJumps.html - explains in detail why 125fps is awesome
http://ucguides.savagehelp.com/Quake3/AquaQuake3Guide.html - the base page that has the above and much more
the links are quake 3 in nature, but apply just the same to open arena and just about all other quake 3 based games.