Osmaneli sits in the Sakarya River valley between Bilecik and Iznik, a small town that became more interesting because modern traffic passed it by. It was once called Lefke and had a large Greek population; silk, river agriculture, and the old road toward Istanbul shaped the place before rail and highways shifted elsewhere.
The reward is a compact old town rather than a headline monument. Walk west from the main road into Camii Kebir and Camii Cedid for timber-framed Ottoman houses, quiet lanes, and the hollow shell of St George Church, the strongest single sight in town. Rustem Pasha Mosque adds the Ottoman anchor, while the Osmaneli old town gives the visit its texture.
Osmaneli is best as a half-day stop if you are moving between Bursa, Iznik, Bilecik, or Eskisehir. It is not a museum town, and some of the historic fabric is fragile, but the mix of river valley, old houses, and post-exchange silence gives it more bite than a normal service town.