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 12,000 and 2,050 years ago, it experienced two caldera-forming eruptions that destroyed the central part of the volcano. The post-caldera eruptions have occurred mostly along the inner perimeter of the caldera from cinder and tuff cones. Here, 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 interpreted as a long-lived 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 that is connected with several shallow anomalies located below the centers of recent eruptions in the caldera. One of such anomalies represents a large shallow reservoir below the Cone A, which was the source of most of Okmok's historical eruptions. The most recent eruption occurred in 2008 and was fed by a magma diapir that was initially formed in the central magma reservoir and then slowly ascended through a ductile silica rich layer at depths from 6 to 2 km. This interpretation is consistent with the petrology studies and modeling of ground deformations.