Unfortunately, the original tombs were completely destroyed by an earthquake that rocked Bursa in 1855 and the structures standing today were built in 1868 by Sultan Abdulaziz in the Ottoman baroque style. The Tomb of Osman is the more ornate of the two tombs and sits on the ruins of the chapel of the Byzantine monastery St. Elie. The Tomb of Orhan was constructed on the floor mosaics of the St. Elie monastery and includes 14 coffins, including those of Orhan, his wife and children.
The city of Bursa is about a four hour bus ride from Istanbul, with buses leaving the main bus station frequently throughout the day. However, you may find it more convenient to join a day tour from Istanbul that will allow you to see many of the main sights of Bursa, including the Tombs of Osman and Orhan.