A problem when you want to expand some items in an NSOutlineView programatically at application start is that the NSTreeController prepares it’s content after awakeFromNib is called on your controller.

To ensure that the data is loaded before you try to expand the items you can observe the content key on the tree controller and expand the nodes as you receive the the observeValueForKeyPath: message as shown below:

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if (object == treeController) {
        // Expand the first row (which is our section header)
        [sourceList expandItem:[sourceList itemAtRow:0]
                expandChildren:NO];
        [treeController removeObserver:self
                            forKeyPath:@"content"];
    }
}

- (void)awakeFromNib
{
    // Listen on the treeController to expand the root node
    // when it has prepared it's content.
    [treeController addObserver:self
                     forKeyPath:@"content"
                        options:0
                        context:nil];
}