An organic light-emitting device including a first electrode, a hole transport region on the first electrode, a light-emitting layer on the hole transport region, an electron transport region on the light-emitting layer, and a second electrode on the electron transport region. The electron transport region includes a first mixed electron transport layer and a second mixed electron transport layer on the first mixed electron transport layer. The first mixed electron transport layer includes a first electron transport compound and a second electron transport compound different from the first electron transport compound. The second mixed electron transport layer includes the first electron transport compound and a third electron transport compound different from the first electron transport compound and the second electron transport compound.