Sorry, I'm a bit unfamiliar with the correct terminology in english...so I might have created some misunderstandings there. (Also I don't have those lower case numbers on my keyboard so I can't write them under the atom symbols as usual

) What I meant was this:
An Oxygen
atom is O. Oxygen (the gas) however consists of Oxygen
molecules which are O2, because one oxygen molecule consists of two oxygen atoms.
Likewise, a hydrogen atom is H, hydrogen gas consists of hydrogen molecules --> H2.
(While the correct terminology may in fact be dihydrogen and dioxide respectively, my german - english dictionary tells me that the gases are still simply called hydrogen and oxygen.)
The correct formula for the reaction of hydrogen and oxygen to water would therefore be:
2* H2 + O2 = 2* H2O (two hydrogen molecules and one oxygen molecule make two water molecules)