Rustem Pasha Mosque is the main Ottoman landmark in Osmaneli, built in the 16th century and still in use. It is modest compared with Istanbul's great imperial mosques, but in this small town that is part of the charm: stone, tile, prayer, and daily life all sit at the same scale.
Visit it together with the surrounding old quarters rather than as a standalone stop. The mosque helps explain why Osmaneli is worth pausing for: not for one blockbuster sight, but for the way Ottoman, Greek, and river-town histories remain visible within a few streets.