What does O & W stand for?

Oldest and Wisest [Nickname for President Ronald Reagan]

Oneida & Western Railroad (IIA)

Ontario & Western Railroad [Nickname: Old and Weary]

Ordinary Wave (MCD)

Oil-dispersed-in-Water [Emulsion]

Oil in Water

Oil-Water [Ratio] [Laboratory science] (DAVI)

Optional With [Automotive engineering]