Just copied this from the files that Pete wrote:
"There are now (1.48 or newer) 5 different OD modes... OD is used
to detect drawings which are outside the front/backbuffer, doing such
stuff in software (or by tweaking polygon coords).
0 (none): fastest mode, glitches in splash screens/game gfx can happen
1 (minimum): takes care of most splash screens
2 (standard): does an easy check, if software drawing/coord tweaking is needed
3 (enhanced): does a more complicated check for soft drawing
4 (extended): does the "enhanced" check, and adds some additional buffer
swap (can cause flickering with some games)
Bye.