Clio,

Obersteuermann did exist in WWI and six of them won the goldenen Preußischen Militärverdienstkreuzes (five for service of U-series boats, the sixt had previously served in one). Oberbootsmann also existed in WWI, they just weren't assigned to submarines often if at all.

The answer would also depend on which of the UB classes...

In general, the crew on an UBIII would be:

CO, WO, LI (sometimes a Marine-Ingenieur-Aspirant and not even a Marine-Ingenieur)

Ratings branches: Engineering: almost always to U-Maschinist (deck officer rank)
Sea Man Branch: Bootsmannsmaat or Oberbootsmannsmaat
Radio: probably not even of petty officer rank
Steuermann: generally, but not always assigned. Sometimes an Obersteuermann. An U-series was more likely to have this more senior rank.

See also [users.hunterlink.net.au]
for ranks.

Best wishes,

Michael