Omar had not gone with Osames Army. (Document)
During his last days, the holy Prophet (peace be upon him and his ahl beit) emphasized and ordered to prepare the Osames army, and said: Prepare Osames army! Damn upon who denies going with it.(Melal-va-Nehal, v.1, p.23 Nahj-ol-Balaghe of ibn Abel Hadid)
However, with all of the holy Prophets (peace be upon him and his ahl beit) order and his damn, Omar and Abubakr did not go with Osames army and stayed in Madine. In fact, they (Omar and Abubakr) were damned by the holy Prophet (peace be upon him and his ahl beit). Rejection of them to go with Osames army is a point that many of Sunnis scholars have mentioned it.