See also: Configure GTK Radiant under Windows
Note: GTK Radiant 1.6 changed its file format, meaning that the OA pack that allows the editor to show OpenArena in the list of available games without having to manually modify configuration files, works with NetRadiant 1.5 and GTK Radiant 1.5, but NOT with GTK Radiant 1.6. You can however use the updated BSPC tool that comes with GTK Radiant 1.6, to compile .AAS files.
Under ..\GtkRadiant 1.5.0\games Edit
Edit q3.game, rename it to oa.game or duplicate and rename.
type="OpenArena" name="Open Arena" enginepath_win32="x:/openarena/" (folder where you have it) engine_win32="openarena.exe" prefix=".openarena" basegame="baseoa" basegamename="Open Arena" knowngamename="Open Arena" unknowngamename="OpenArena modification"
Under ..\GtkRadiant 1.5.0\q3.game Edit
Edit q3.game (the folder name), rename it to oa.game or duplicate and rename.
Baseq3 -> Baseoa
Replace the Q3 shader list with OA shader list:
common detailtest oasky beams liquid_lavas liquid_water oalite oaflares oanew oa_fogs quark evil8 clown clown2 mc-oa terrain oasfx nki islandctf suction czest
Note: if textures in the texture browser do not load, try copying the above shaderlist to the openarena/baseoa/scripts/shaderlist.txt file, creating the file and scripts/ directory, if they do not exist. To reiterate, if textures still do not load in GtkRadiant add the shaderlist.txt in the baseoa/scripts directory in your OpenArena GAME path (i.e., C:\OpenArena), NOT the GtkRadiant oa.game/ path.
Running a map Edit
To avoid the "not found" error when trying to load your map from the console, go in the OA cfg file (it's in your windows user account, application data folder) and add seta sv_pure "0"
Don't forget that your map must be in a /baseoa/maps/ folder. I have had a bit of trouble with no textures being loaded in game until I noticed that my map was in /baseoa/ folder.
Another warning: ever wanted to kill radiant for not loading your map and/or compiler complaining that map file could not be found? If you look at radiant's console and see some parse errors, open your map in notepad and replace all texture references with some other random, not special, not common shader, texture. In case you didn't texture your map yet, it seems that the default texture picked by radiant cannot be used, so to prevent this, just map using some wall, floor or caulk texture.