If you are looking to do a hydro-test calc., assume the o-ring essentially fills the groove, i.e. the contained pressure is felt at all groove surfaces.
Your question is very vague. There is a gradient in contact pressure depending on location on the o-ring. The only requirement for a good seal is that the peak contact pressure is greater than the fluid pressure. In a well designed seal the average contact pressure is not much greater than the fluid pressure.
