To do Alt + P O on Mac in Excel, download and install Accelerator Keys, then press Option, followed by P and O.
Command: Orientation
This is how to do Alt + P O on Mac in Excel with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with Excel.
Alt + P O opens the Orientation menu to switch between Portrait and Landscape page orientation for printing. Portrait is taller than wide, while Landscape is wider than tall, useful for spreadsheets with many columns.