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Using CIP rules in bicyclic compounds

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.)