The Olive-sided Flycatcher has an olive-grey or olive-brown head, back and flanks. Wings and tail are darker. Belly, chest and throat are dull white. The female and the male are identical! It measures about 20 centimeters, has a wingspan of about 33 cm and weighs about 35 grams, almost the weight of a clementine! He lives about 10 years.

This bird is mainly insectivorous, it mainly eats flying insects like bees and wasps. Like the other birds of its family, the Tyrannidae, it awaits its prey on a perch and flies up to capture it in flight when it spots it. Nesting males and females aggressively defend their nest by swooping down on potential predators like other birds and squirrels and clicking their beaks.