The behavior of these key combinations when editing the contents of an e-mail message has recently changed, making normal whole-word text selection behavior on the Mac inaccessible. Option+Shift+RightArrow used to extend selection to the end of the word to the right of the cursor, as it normally does in other Mac software, but lately it has been re-mapped to "indent paragraph," presumably because the Option key also serves as an Alt key for Windows-oriented software, and Alt+Shift+RightArrow has that meaning in MS Office products under Windows.
For those of us who are heavy keyboard users, this is a severe compromise. Imagine not being able to use Ctrl+Shift+{left/right-arrow} to select words under Windows. I notice that the keys work correctly in THIS formatted editor on the forum, by the way; just not in Outlook 365.