After a few months of rest, he was placed in charge of the Third Army in Rumelia. In 1873 he became a Lieutenant General and returned for a short while to Constantinople, before being sent to Iskodra and later to Bosnia. His appointment there didn't last long because he couldn't get along with the local governor, Derviş Pasha, so he was moved to the Fourth Army. In 1876, the Principality of Serbia proclaimed its independence and declared war on the Ottoman Empire. Osman Pasha, who had at that time his headquarters at Vidin, defeated the Serbian Army, but in April 1877 Russia declared war on the Ottomans. The Russian troops crossed the Danube into Bulgaria and Osman, with his army of 15,000 men and 174 cannons, was tasked with protecting the important fortress of Nikopol. Before he could get there, the city fell on 16 July after the Battle of Nikopol.