A sales rep who owns an opportunity record will have read-only access to the related account record, if the organization-wide default for the Account object is private. This is because the opportunity owner is automatically granted read-only access to the account that the opportunity is associated with, regardless of who owns the account. This is called implicit sharing, and it is a built-in feature of Salesforce to ensure that users can access the data they need to do their work. However, the opportunity owner will not be able to create, edit, or delete the account record, unless they have other sharing mechanisms that grant them higher access, such as role hierarchy, sharing rules, manual sharing, or View All or Modify All permissions. References:

[Salesforce Certified User Experience Designer Exam Guide], Section 3.1: Design and implement solutions that meet user needs

[Control Access to Records Unit | Salesforce Trailhead]1, Unit 2: Implicit Sharing

[Work with Related Lists on Records in Lightning Experience - Salesforce]2, Related List Cards