Yep, the CR for 1 ogre is 2, so the EL (Encounter Level which is what I assume you mean) should be 4. Start with the CR of one critter, and add 1 for each time you double the number of critters. That's the rule, I think, anyway.
Actualy the rule of thumb is increase the EL by 2 for each doubling. This is why 2 ogers are 4 (1x2, 2+2) and four (2x2, 4+2) are 6. It would take 8 ogers to get a EL and 16 to make a EL 10 encounter. Now this does break down after a point.