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

[![bicyclo[2.2.1]heptan-2-one][1]][1]


  [1]: http://i.stack.imgur.com/9yywJ.png

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.)