OpportunityFieldHistory details

 List<OpportunityFieldHistory> ofh  = [Select Id, CreatedById,
 CreatedDate, Field, NewValue, OldValue from OpportunityFieldHistory
 where OpportunityId = :op.id];

  System.debug(ofh);//this was giving the complete details
  System.debug(ofh.oldvalue);//here i git the error called variable
  doesn't exist :oldvalue