My org account object OWD is private. And one of my Sales Rep. try to make an opportunity record and in lookup he wants the account record which he doesn't own. So , can he create opportunity related to that account or not...?
No, they won't be able to create an Opportunity that looks up to that Account. Without access to the related record, they'll receive an 'Insufficient Privileges' error even if they manage to add the Account Id to the lookup.
If they've got a legitimate need to access that Account, then there are a number of options to provide access, such as the Owner manually sharing the Account or sharing rules. You'll need to assess your options and I'd suggest starting with the Sharing Settings help article or Trailhead to help.