The Oglio (Italian: [ˈɔʎʎo]; Latin: Ollius or Olius; Lombard: Òi, Cremonese: Ùi) is a left-side tributary of the river Po in Lombardy, Italy. It is 280 kilometres (170 mi) long. In the hierarchy of the Po's tributaries, with its 280 kilometres (170 mi) of length, it occupies the 2nd place per length (after the river Adda), while it is the 4th per basin surface (after Tanaro, Adda and Ticino), and the 3rd per average discharge at the mouth (after Ticino and Adda).

The Oglio is formed from the confluence of two mountain streams, the Narcanello branch from the Presena Glacier, in the Adamello group and the Frigidolfo branch, in the Corno dei Tre Signori, part of the Stelvio National Park. The streams merge near Pezzo di Ponte di Legno, both the streams have an average discharge of 1.5 m3/s (53 cu ft/s). The Frigidolfo branch, before merging with Narcanello branch, receives the Arcanello branch, which have an average discharge of 0.5 m3/s (18 cu ft/s), which receives a minor branch originating from Lake Ercavallo.