In bicyclo[2.2.1]heptan-2-one, arrange the numbered groups according to CIP rules:


Group 1 will have highest priority because oxygen atom is attached to 1. (It can be considered as two oxygen atoms). 

Group 4 will have least priority due to least atomic number among the numbered groups.

But I am confused about order of priority of group 2 and 3. Both have same atoms. (So I will check the next atom which is also same.)