OMAC was a company based in Milano, Italy. It was owned and run by Alfredo Cardinali. It is possible that, like Wissner, its main product was bicycle bells.
OMAC may stand for something like Officine Meccaniche Alfredo Cardinali - which may translate as 'Mechanical Workshop of Alfredo Cardinali'. A lot of small engineering businesses in Italy seem to have names composed of 4 initials with the first two being 'OM'. For example, weight-weenies of a certain age will remember the 'Big Sliding' components made by OMAS.
In the late 1940s OMAC produced a rod type derailleur with a single, spring loaded pulley on the end of it. Neat but bizarre.