Explanation
Disabling "Grant Access Using Hierarchies" and disabling the "View All" permission on all other profiles are two options that would ensure access only to the record owner and the administrator for a custom object with private sharing settings. Disabling "Grant Access Using Hierarchies" would prevent users above the record owner in the role hierarchy from accessing the record. Disabling the "View All" permission on all other profiles would prevent users with other profiles from accessing all records of that object. Disabling the
"Create" permission on all other profiles would prevent users with other profiles from creating new records of that object, but not from accessing existing ones. Disabling the "Read" permission on all other profiles would prevent users with other profiles from accessing any record of that object, but also prevent them from creating or modifying records.