Officers say the incident happened at 11:35 p.m. Tuesday on the 400 block of N. Main St.
The bike's owner was riding north when the suspect approached him. The owner told police the suspect told him to give him the bike. When the owner refused, the suspect hit him in the face and took the bike.
Police say the bike's owner was injured, but refused treatment.
Oshkosh police are asking anyone with information about the incident to call them at (920) 236-5742.