A military mortar was discovered in Cherwell River, in Christ Church Meadow, by two teenage boys on a fishing trip.
A member of the public alerted the police at 3:46pm yesterday, and the mortar was destroyed at 5pm today.
A large area around the site, including much of Christ Church Meadow, was closed off but has now been reopened.
Investigating officer Ch Supt Andy Boyd, head of Thames Valley Neighbourhood Policing, said: “I am very grateful to the members of the public who helped to share our appeal, and for spreading the message about this incident.
“I am also grateful to the two boys who helped us locate this item and ensure that it was safe and that no members of the public are at risk.
“I understand that some people who wished to use the park today have been inconvenienced, and I would like to thank them for their ongoing understanding and patience.”
Multiple rowing taster sessions were disrupted today as a result of the closure.
