The term "Ost" (= east) refers to the origins of the men who fought in these formations: they were recruited from amongst the population of the occupied eastern regions (Baltic states, Byelorussia, Ukraine, Russia etc.) and from the Soviet prisoners of war in German hands.
Amongst the people to be found within Soviet Union, there were many with nationalist and/or anti-communist tendencies, and many of them would have been happy in 1941 to join in arms with Germans against Soviet Union. However, the official German policies of the time were not exactly aimed at taking advantage of such tendencies (being actually much better in alienating the conquered population against their conquerors), and it took time before actual Ost units would start to be formed.
Besides the eastern front, they would also see service in the Balkans, Italy and France. Some of them might have taken part in defending Normandy during the D-day. Wikipedia has information on a number of individual units and their histories:
http://en.wikipedia.org/wiki/Ostlegionen
Besides the normal Ost infantry units, the game OOB also provides for Ost SS infantry units and Ost cavalry. Waffen SS was more open to having foreign volunteers serving in their ranks than Heer, which was rather restricted to (ethnic) Germans.
As for cavalry, there was at least one rather sizeable formation recruited from amongst the Ukrainian cossacks. This ended the war surrendering to western allies somewhere in northern Italy or Austria, but they were eventually turned over to the Soviets.
It might be worth remembering that in the Soviet military, ending up as a prisoner of war in the enemy hands was considered a crime, which could get you shot, sent to a labour camp or (during the war) to a penal battalion. Men who had actually willingly volunteered for service under the Germans could expect at least equally harsh punishments if they would end up in the hands of NKVD.