First it isn't written
2H2 + O --> 2H2O but as
2H2 + O2 ==> 2H2O for the FORMATION of water.
Whereas, H2O ==> H2O isn't telling you how the water was formed from the elements.
You could have something like
H2O(l) ==> H2O(g) where l stands for liquid and g stands for gas.