In 1881, 14 dōtaku were discovered on Mount Ōiwa, and ten more were found in two separate locations in 1962 during construction work on the tracks of the Tōkaidō Shinkansen.

Of these tumuli, the Maruyama Kofun (円山古墳), a "two conjoined rectangles" type (zenpō-kōhō-fun [ja] (前方後方墳)) and Kabutoyama Kofun (甲山古墳), a circular-type [ja] (empun (円墳)) which has the largest stone burial chamber in Shiga Prefecture, were designated as National Historic Sites in 1941.

The Toba Kofun (冨波古墳), another zenpō-kōhō-fun [ja] was discovered in 1982 and added to the National Historic Site designation the same year.

In 1985, six previously unknown tumuli were discovered and were also added to the area under protection, which then received the collective name of "Ōiwayama Kofun Cluster".

[2] Artifacts uncovered during archaeological excavations have included bronze mirrors, earthenware, haniwa and Sue ware pottery.