You can use InDesign's GREP styles for that.
First of all, create the character style "option", which is set to your needs (if it doesn't already exist).
Modify the TOC format such that a right aligned tab (
\y) is used before the page number instead of a usual one (
Then modify your TOC-entry's paragraph style (if you're using the same as the original paragraph, you need to copy the style, optionally with the original as its base) and select "GREP styles". Here, add a new entry using the "option" format to anything matching
(?<=\D\t).+(?=~y\d+$) (click the link for an explanation, except that we're using
~y to match the right-aligned tab instead of a regular tab). Also remove the regular tabulator position of this format!
The RegEx above will match anything that your title has after a tab. Alternatively, you can of course simply have it match
\(opt\) (the parentheses must be
\-escaped in RegEx).