CoNi coin in very good shape, F quality   (see table of abbreviations)

 Dimensions and weight: Ø 21 mm  -  3 g (30 g with shipment packing)

A little history:

Ober Ost, short for Oberbefehlshaber der gesamten Deutschen Streitkräfte im Osten (meaning Supreme Commander of all German forces in the East), is a German term used during the First World War. In practice, in addition to the person responsible for its administration, this title refers to the staff of the high command of the German army's military occupation authority responsible for administering part of the territories of the Russian Empire conquered on the Eastern Front between 6 June 1914 and 26 December 1918, as well as these territories themselves.

(source:  Wikipedia)