On nomenclature of certain compounds I have some questions on the nomenclature-
(1) In $H_2O$ we have $H$ written first and $O$ is second but when it comes to Hydroxyl group $OH$ the $O$ comes first, Why?

(2)Why is Propanal called Propion-aldehyde and not Propane- aldehyde?
To answer (2) first, propionaldehyde is the common (Trivial) name and hence is derived and held-onto only due to historical reasons and now obsolete nomenclature reasons. The 3-C aliphatic saturated qroup was historically called Propion- and hence propanoic acid corresponds to propionic acid and propanal corresponds to propionaldehyde, and propanoate(ester) corresponds to propionate in trivial nomenclature. It is a consequence of the history of the original naming of the 3-C aliphatic group at the time of its discovery.
As for (1), it is a consequence of pure convention; just as water is not called hydrogen hydroxide!