Why is '/opt' hidden, and what can I do to unhide it?
Out of the box, there is no /opt file delivered by OS X.

This command will show the hidden status of files in root ls -lO / with O being the capital letter, not zero. The chflags command can be used to toggle the hidden flag if needed.

Depending on how you got /opt created, you may or may not have issues with the permissions changing again - most tools leave things alone with respect to filesystem flags as long as the read/write/execute permissions are not removed.