On a PC this is Alt+O,C,A, as explained here.
Is there an equivalent keyboard shortcut in Excel for Mac, to fit the column width to the width of the text?
I'm using the 2011 version.
Although Microsoft did not include many keyboard shortcuts in their Office Suite for Mac, you can make your own for any app including Office apps. To do so:
In this case you must type in AutoFit Selection just as it appears on the menu in Excel. (Format > Column > AutoFit Selection)
You can choose any key combination that suits your fancy.
