@interface OutlineViewController : NSOutlineView <NSOutlineViewDataSource, NSOutlineViewDelegate>
@end

@implementation OutlineViewController
- (CGFloat)outlineView:(NSOutlineView *)outlineView heightOfRowByItem:(id)item
{
if (myCondition) { /* ... */ } else {
     // how to return default result from here
     // return [super outlineView:outlineView heightOfRowByItem:item];
}
}
@end