A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Once you click On My Mac then navigate to the folder & open a file it should be persistent unless you later navigate to or open a file in a different location.
However, if you have a specific folder you want to be able to return to readily just add it to the Sidebar in a Finder window. See Mac Help on the subject:
Customize the Finder sidebar on Mac