Is this equation correctly balanced 2 O2 H2O?
This is not an equation at all.
An example of a correct (and balanced) equation would be:
2H2 + O2 --> 2 H2O
2H2 + O2 ---->2H2O is balanced for the following reasons. Hydrogen and oxygen exist as H2 and O2 and not H and O. The product is water with a formula of H2O. To get balance the number of atoms of reactants and the number of atoms of products must be equal. They are in the balanced equation above. For these reasons the equation is balanced.