I have read those overview-tutorials.

But when I go to the overview mode, the map is just grey and everything outside the map is just green. I've heard it's because of some bmp options are wrong...

I know nothing about that. So would someone explain how I can make it work properly.

make sure fog is off

Check for any error messages in the console on loading the overview map.
My guess is that you forgot to use video resolution of 1024x768.

cl_particlefx is 0 (I guess it means that fog is off)

and I am using resolution 1024x768.

A weird part was that a few entities were seen as they should... three of them I think. But everything else was just gray.

cl_fog 0 ..not sure if cl_particlefx 0 will do it

Alright, I was able to get the fog off and the ground looks right now. Thanks to Sagitarius.

But the entities are not visible. (I tried r_drawentities 1 but it did nothing) How can I make entities visible?

And one other thing... How can I rotate the overview? None of the tutorials explain it.

The overview doesnt rotate, thats only in hl, cs, not in dod.

you need to do that to the image. youll need to take hte image at and angle and then rotate the .bmp. but then it wont line up which is the entire point so i really suggest you dont.

Originally posted by Tiberian
But the entities are not visible. (I tried r_drawentities 1 but it did nothing) How can I make entities visible?

Two thing to try for that one.

compile using -fast for VIS or don't even bother with VIS and RAD.

It all depends on how big your map is :)