There were several events  in the history of the Ottoman Empire that can be considered turning points, but one of the most notable was the Battle of Vienna in 1683. 

  The Ottoman Empire expanded its territory over the centuries and from the 16th century became one of the most powerful empires in the world. But in the 17th century, the empire began to decline due to a combination of economic, political and military factors.

  
 In 1683, the Ottoman Empire, led by Grand Vizier Kara Mustafa, began a large-scale siege of Vienna,  then the capital of the Habsburg Empire.  Ottoman troops were confident of victory, but  were eventually defeated by a coalition of European troops led by the Habsburgs.  

The Battle of Vienna marked the beginning of the end of the expansion efforts of the Ottoman Empire in Europe. It also weakened the Ottoman military and undermined the confidence of its rulers as they began to adopt more defensive strategies and focus on consolidating their existing territories rather than expanding them. 
 

 The Battle of Vienna also marked a shift in the balance of power in Europe, with the Habsburgs and their allies emerging as the dominant power. This paved the way for the rise of other European powers, including Prussia and Russia, which would later challenge the dominance of the Ottoman Empire in the region.  

Overall, the Battle of Vienna was an important turning point in the history of the Ottoman Empire and had far-reaching effects on Europe and the rest of the world.