It is the responsibility of the Event Manager to identify risks and take steps to mitigate those risks. One of those risks is security, particularly if you have VIPs attending or the event may attract controversy.
Oxford University Security Services will be able to advise you whether your event warrants security, and if so, will be able to assist you by providing advice and help you put together a security plan. Early notification to Security Services will allow for a full security risk assessment to take place. This may include liaising with outside agencies such as the police, a process you will be guided through.