If OWD of Opportunity is set to private, then which all opportunities can a user view?

Answer: See the explanation.

Explanation:
As OWD of Opportunity is set to private, so any user will be able to view only these opportunities:
1. Opportunities owned by the user.
2. Opportunities shared with the user using Opportunity Sharing Rule.
3. Opportunities owned by users below this user in the organization role hierarchy as ‘Grant
Access Using Hierarchies’ is always checked for standard objects. It cannot be unchecked.
4. Opportunities on which this user has been added as an Opportunity Team member