If you also want to return records where Oppo_stage != 'Bus_Written' (wthout any other checks for date (closed)) then try this:

Select

oppo_opportunityref

,oppo_stage

,oppo_closed

From

Opportunity

Where Oppo_stage != 'Bus_Written'

OR (

Oppo_stage = 'Bus_Written'

AND (oppo_closed is null or Oppo_Closed >= GETDATE() -7)

)