The Olive-sided Flycatcher breeds in the montane forest of the Rockies and the coniferous forest of North America. It migrates during the northern winter to Central and South America between September and May. See the distribution map at Birdlife International.

It captures its insect prey by sallying usually from an exposed perch and often returning to that same perch many times.

It has a large head and a short tail compared to similar species such as the pewees. The throat and centre of the underparts are whitish while the olive grey flanks extend across the breast giving the impression of an unbuttoned waistcoat. Note also the barring on the crissum which is always unbarred in the case Tropical Pewee C. cinereus.