1WO means...

[frau kaleun]

Originally Posted by JScones

There are no direct equivalents to Fähnrich and Oberfähnrich in any non-German based armed force. They are strictly neither ensigns or midshipmen in the US or UK sense. They can be, however, considered a subset of midshipman for the purpose of easy explanation.

Yeah, there's really no term/rank that matches up exactly. "Midshipmen" just seems like the handiest term to use to indicate their relative status (at least at Fähnrich or above) when serving on a u-boat.