As regards the Osterknacht - they are (were?) the premier knightly order of the Ostermark. In essence, as I understood it from Claws of Chaos, they are something like the Reiksguard is to the Reik, only for the Ostermark. Same colors (Maroon and Yellow) as the Province, headquartered in the capital, etc.
However, the Claws of Chaos is set during the Time of Three Emperors, so what the Order's been up to since then is anybody's guess. It's probably safe to say that they're still around, though.