Every risk needs to have a single owner. That is not necessarily the person who is going to carry out all the mitigation. It is the person who is responsible for ensuring that the mitigation happens, and who answers to the Board or project manager for the risk. It is no good assigning risk ownership to someone who is not present, as they are unlikely to accept it. Every risk should be owned by someone who is round the table and part of the risk discussions. If you don’t have the right people round the table, get them there.