I’ll address your question more broadly. The German navy had until 1938 something called a Deckoffizier, which is best translated as a warrant officer. (The German navy had much more of the distinction between commissioned and warrant officers than say the Royal Navy.)

Steuermann and Obersteuermann were both Deckoffizier ranks, navigation branch warrant officers. You can think of an Obersteuermann as a chief warrant officer, a Steuermann that had done his job for a few years and been promoted.

The other Deckoffizier rank that commonly served on U-boats in WWI were engineers, with the ranks called Maschinist and Obermaschinist. For smaller U-boats, these were often the boat’s chief engineer. For larger U-boats, they were the second engineer.

All Deckoffiziere were promoted from the ranks after completing training at the Ingenieur- und Deckoffizierschule at Wlhelmshaven. Obviously, only the best petty officers were considered for promotion to warrant officer. Time in service before becoming a Deckoffizier ranged considerably. For a U-boat Steuermann prewar, about 11 to 15 years was typical while for Maschinists it was more like 8 to 12 years. This went down during the war.

In addition to the regular navy, the German navy also had reservists of varying types. A substantial portion of the Steuermänner and Maschinists were reservists, and these would have seen merchant marine service especially prewar. My understanding is that a portion of the reservist Steuermann on U-boats were junior merchant marine officers taken into naval service but I don’t have all the details on how that worked.