I have a question

Currently we have 2 BU's 

Business Unit A 

Business Unit B

John under BU A - he is able to VIEW BU's B records -but I also need him to be able to edit records (Opportunities). 

I'm somewhat unfamiliar with the security and have tried to figure out the difference with other roles that can -but it's just confusing me more. Can someone shed some light on this? I exported this role to Excel. Sales - Read-only and Append role(Entire Organization).xlsx

Maybe it just needs a tick in Opportunity under "Append"?