The gate and some stone walls from this castle ruin can be seen from across the river, but not actually visited. It is located in an off-limits part of the Naha Military Port.
This is what JCastle has to say about the history of this castle:
It is unknown exactly when this port fortress was built. However, Omono Gusuku can be found on the Ryukyu Kokuzu (琉球国図), a map dated to around 1452. Omono Gusuku along with Mie Gusuku and Yarazamori Gusuku guarded Naha Port during the time of the Ryukyu Kingdom. Unlike the other two forts, which guarded the entrance into Naha Port, Omono Gusuku functioned mainly as a customs post and storage area for traded goods and weapons.