What would happen if O2 were added to N2(g) plus O2(g) 2NO(g) at equilibrium?

Answer 1

More N2 and O2 would form

Answer 2

At equilibrium the equation is

N2(g) + O2(g) 2NO(g)

two moles of reactants form 2 moles of products.

Now if you add more moles of reactant gas, the system will react in such a way as to minimise the disturbance of adding more moles. So it will form more moles of NO. However, the position of the equilibrium will be different.

This follows Le Chetelier's Principle, which deals with equilibria.