Poster submarine Oyashio class

The Oyashio class is a series of conventional attack submarines, introduced into the Japanese Navy in 1998, with 11 units in service. Based on the previous class, Harushio, the Oyashio class is distinguished by a cigar-shaped hull, necessary to integrate a side-scan sonar. This sonar, combined with a towed sonar, improves submarine detection, although it slightly reduces the hydrodynamic efficiency at high speeds, little used by conventional submarines.

The construction of the Oyashio submarines also introduced a sound-absorbing material, making the submarines more stealthy. The engine, an optimized Kawasaki V12, and the reinforced batteries compensate for the performance losses due to the hull shape. Advanced automation, as well as the gradual addition of improvements such as a CO₂ capture system and a deployable antenna, have increased their efficiency.

In 2011, Japan decided to increase its active fleet by extending the life of seven Oyashio submarines to 24 years (compared to 18 for the Harushio class). In 2018, the Ministry of Defense confirmed the completion of this extension program, estimating these submarines to be almost at the level of the Sōryū class. The first two Oyashio were converted into training submarines.