Limiting reactant question??? please help what am I doing wrong?

So the question says NH3 + O2 --> NO + H2O

So I balanced the equation and got

4NH3+9O2 --> 6NO3 + 6H2O

I converted grams to moles and then multplied by the mole ratio

so I got 3.25 g of NH3 will produce .286259 moles of O2 (didn't say which one to use so I just picked water)

For O2 I got 3.5 grams produces .65625 moles of H2O

HOWEVER the answer says the limiting reactant is O2. How can this be if NH3 produces less water?

please explain or work out if possible

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