How to fold output cells by default?

I'm slow, but eventually I get there. ;-) You need CellEpilog to make the behavior automatic on evaluation of any cell. Importing the code from my autoFoldOutput function:

SetOptions[EvaluationNotebook[], 
 CellEpilog :> 
   (SelectionMove[EvaluationNotebook[], All, EvaluationCell];
    FrontEndTokenExecute["SelectionCloseUnselectedCells"])
]

Simply evaluate that in your Notebook and all output groups will be automatically closed when they are created. You may replace EvaluationNotebook[] with $FrontEndSession if you want to affect all Notebooks during the session or with $FrontEnd to persistently affect all Notebooks.

Not sure if this is what you're looking for, but the Cell menu has an item that collapses open cells:

Here I highlighted the open input/output cells and then collapsed them to get the following:

If your question is about automatically and immediately collapsing every output cell, then that's a different story. It will most likely involve this answer.