Some boats came to the Japanese Navy from Germany.

O-4 was UC-90 and O-5 was UC-99

Builder Blohm & Voss (Hamburg)
Displacement 491/571 tons
Length 56.5 meters (185’ 5”)
Beam 5.5 meters (18’ 2”)
Draft 3.8 meters (12’ 4”)
Machinery twin-shaft M.A.N. 6 cyl, 4 stroke diesels two SSW electric motors
Power (diesel) 660 hp
Power (electric) 770 hp
Speed (diesel) 11.5 knots
Speed (electric) 6.6 knots
Fuel 77 tons
Radius (diesel) 9,850 miles @ 7 knots
Radius (electric) 40 miles 4.5 knots
Tubes (side mounted) two 19.7 inch and (aft) single (carried seven torpedoes)
Tubes (mine stowage) six (fwd) (carried 14 mines)
Guns single 10.5cm/45 cal deck gun
Crew 32 men

These belonged to the UC-III TYPE mass-produced towards the end of the war. They had six mine tubes in the forward portion of the hull. In spite of their small size, they were double-hulled. They had two torpedo tubes amidships, external to the pressure hull, outboard of the conning tower, and another in the stern. The external torpedo tubes increased the drag, contributing to the low surface speed, and were considered to be an inferior design. I.J.N. did not show much interest in this type, as it was regarded to be too small to be used in the wide Pacific.

O-6 was UB-125 and O-7 was UB-143

Builder AG Weser (Bremen)
Displacement (UB-125) 512/643 tons
Displacement (UB-143) 523/623 tons
Length 55.9 meters (183’ 4”)
Beam 5.8 meters (19 ‘)
Draft (UB-125) 3.7 meters (12’ 2”)
Draft (UB-143) 3.8 meters (12’ 3”)
Machinery twin-shaft M.A.N. 6 cyl, 4 stroke diesels and two electric motors
Power (diesel) 1,060 hp
Power (electric) 788 hp
Speed (diesel) 13.9 knots
Speed (electric) 7.6 knots
Fuel 78 tons
Radius (diesel) 7,280 miles @ 6 knots
Radius (electric) 55 miles @ 4 knots
Tubes (fwd) four 19.7 inch and (aft) single (carried ten torpedoes)
Guns single 8.8cm/30 cal deck gun
Crew 34 men

These small submarines belonged to the UB-III Type to be used for coastal operations. They were slow, but well-balanced and excellent commerce-raiders. I.J.N. however, did not pay much attention to them.

What I.J.N. desired most of all was the U-Cruiser of the U-139 class. A submarine of this type was allocated to the U.S., Britain and France but Japan was denied one. It was known that Germany had designed an even more advance U-Cruiser; U-142 but it was not built yet. Mr. Matsukata, President of Kawasaki Shipbuilding Company, traveled to Europe to obtain the blueprints of U-142 and he did succeed in his mission. The early I-Boats were based on German designs, and the German engineers invited to Japan rendered valuable assistance to the building of new submarine types.