OnPreRender is called before a camera starts rendering the scene.
This function is called only if the script is attached to the camera and is enabled.
Note that if you change camera's viewing parameters (e.g. fieldOfView) here, they will only take effect the next frame. Do that in OnPreCull instead. OnPreRender can be a co-routine, simply use the yield statement in the function.
Also note that when OnPreRender is called, the camera's render target is not set up yet, and the depth texture(s) are not rendered yet either. If you want to do something later on (when the render target is already set), try using a CommandBuffer.
See Also: OnPostRender.