The web is not an OS. The web is a layer that operates through an OS.
The distinction is important to make, because it demonstrates both the web's greatest advantage (the fact that it can reach across many computers at once) and its greatest weakness (it will always be slower than the desktop equivalents).
... Just like an application written for an O/S will always be slower than a custom written application which runs straight on the hardware with no O/S.
It depends on your definition of O/S, and what you think it should do.
The distinction is important to make, because it demonstrates both the web's greatest advantage (the fact that it can reach across many computers at once) and its greatest weakness (it will always be slower than the desktop equivalents).