Of course, there will not be a 1/1 correspondence between HRE and Empire provinces. However, the description of Ostermark as "a vast and open land, essentially flat, and dominated by fields and verdant pastures," does not fit Bohemia. Kislev seems a mixture of Bohemia, Poland and Hungary - countries which were all ruled by the Jagiello dynasty around 1500.
If the name is anything to go by, Ostermark quite obviously is a contraction of Ostarrîchi and Ostmark, both referring to of the Margraviate of Austria, part of Bavaria in the early Middle Ages, which later was elevated to the Duchy of Austria. The former is the Old German name (still in use as Österreich), and the latter a 19th century translation of the Latin Marcha Orientalis.
I would rather assume the Hussite Campaigns to be comparable with the Chaos invasions.