To allow a volunteer to create opportunities in your organization, grant them the Opportunity Manager permission. To learn more about changing a volunteer’s permission level, read Change a Volunteer's Permissions.
Note: When you make a volunteer into an Opportunity Manager, they do not automatically become an Event Manager in existing opportunities or in opportunities created by other users. If they must access specific Opportunities that someone else created, they will need to be added and granted the Event Manager role manually. If the user needs to have access to all Opportunities, then full Organization Manager permissions may be the route to take.