Okmok volcano located on the northeastern part of the Umnak Island is one of the most active volcanoes in the Aleutian Arc. It was initially built as a large shield volcano, but was strongly destroyed by two caldera-forming eruptions 12,000 and 2,040 years ago. The post-caldera eruptions occur mostly along the inner perimeter of the caldera from a series of distinct cones. Here, we perform seismic tomography to explore the deep sources of magmatic activity beneath Okmok. We use the local earthquake data of the Alaska Volcano Observatory (AVO) in the time period from 2003 to 2017 to build a model with the 3D distributions of the P and S wave velocities and Vp/Vs ratio. At depths of more than 10 km, we observe a vertically aligned anomaly of high Vp/Vs ratio representing a steady conduit likely responsible for the volcano evolution since its origin. Above this conduit, we reveal a large anomaly of high Vp/Vs ratio representing the main magma reservoir providing the material for all recent eruptions in the caldera. It appears to be connected with another large shallow reservoir located below the Cone A that was the source of most of Okmok’s historical eruptions. The most recent eruption in 2008 took place right above the deep conduit. To reach the surface, the magma for this eruption passed through the shallow ductile zone, where it was saturated by silicic components. This interpretation is consistent with the petrology studies and modeling of ground deformations.