State one major difference between having a nitrogen atom in a ring structure and having an oxygen atom in that structure

Hello!
One major difference between having a Nitrogen atom in a ring structure and having an oxygen atom in that structure would be that Oxygen atoms are electron attractors by resonance and Nitrogen atoms are electron donors by resonance. 
In a ring structure, Nitrogen can donate an electron and become positively charged and Oxygen can attract an electron from the ring to become negatively charged, as electrons relocate in resonance structures. This can produce changes in terms of reactivity and stability of the ring.
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