To the best of our knowledge, all caps is the consensus in the industry for a character extension next to the character name (parentheticals below the character name are lower-case).
There are nevertheless a couple of ways you can have lower-case character extensions. Go to Document > Mores and Continueds and change the text in the "Top of Next Page" field to lower-case:
The character continued inserted when a piece of dialogue is separated by a page break will now be lower case.
If you want a lower-case "o.s.", rather than typing in the open parenthetical, right-click the mouse over the character name on Windows or Control + click on Mac. You'll get a contextual menu with a lower-case "o.s." as an option: