According to the PMP, the risk owner can be any project participant. He can be the project manager, a project stakeholder, the project sponsor, a project team member, etc...
The risk owner has the following responsibilities:
- He's accountable for the risk in front of the other stakeholders
- He must devise the proper response measures for the risk he can that risk materializes
- In short, the risk owner is responsible for managing the risk
Risk owners are selected based on their knowledge/experience with the particular risk(s) they're owning.
Note that the project manager must include risk owners of each and every risk in the risk management plan of the project, in particular in the risk register.
PS: The project manager is always responsible for following up on the risk though with the risk owner.