Cam Sensor Seal - O Ring size? 4.6 thor

I have a pile of different size o rings that came with a rebuild kit I bought from Island 4x4. I dont know which is the correct one to use? I started of with the wrong one and oil ****ed out all over. I would like to get it right this time and need help. The sensor OD is 18mm and the chamfered bore that it presses into is about 26mm.
Please help.. I need to spend time fixing other problems and not cleaning up oil..
Thank you. Tom