How expand first root node when NSOutlineView is done with loading

It's unclear what you mean by "loading all data" here. A NSOutlineView is populated with cells synchronously, although it's actually a bit more complicated that than:

— Cells are only created for visible rows (and perhaps a couple of additional rows at the top/bottom of the table).

— The UI update that results from populating the table typically occurs on a subsequent cycle of the main even loop, so it's "a little bit" asynchronous.

— If there is some complexity in configuring cells, you might be seeing delays because it's done synchronously.

— If your custom cell content needs to be fetched from (say) a network, you introduce your own asynchronous delays into getting cell content displayed.

So, what is it that you actually want to wait for here?