To do Alt + H B O on Mac in Excel, download and install Accelerator Keys, then press Option, followed by H, B, O.
Command: Bottom Border
This is how to do Alt + H B 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 + H B O bottom Border applies a border to the bottom of selected cells. This is one of the most common border styles for separating rows of data.