From over + -ling on analogy with underling, or as a revived or dialectal term from Middle English overling, equivalent to over + -ling.

overling (plural overlings)

1. (rare) A superior.
   • 1917, Rudyard Kipling, Mesopotamia (poem)

     But the idle-minded overlings who quibbled while they died, [...]

   • 1973, Thomas Pynchon, Gravity's Rainbow:

     Reflex laughs from the underlings present, though only grim smiles from the overlings.

2. (UK dialectal) A ruler; master.

• underling