Unable to find default tag file C:/Program Files/GtkRadiant 1.5.0/oa.game/shadertags.xml. No tag support.
ignore this message or create a tag file and it will go away and still be a useless feature.
14:9: parse error at 'brushDef': expected '#quake3-primitive'
brush 0: parse error
entity 0: parse error
QuArK saves maps in a slightly different format, a format that radiant 1.5 can't read. You can always use Q3Radiant to convert the map to a readable format, but then you have to deal with textures not aligned and with brushes that have been resized.
Or you can suck it up and use the POS QuArK map editor.