[Celjabba]

A good exemple would be Toph from Avatar:The Last Airbender
She goes bare footed, covered in dirt, being a slob and swearing with the best of them - by choice. She is still a well educated child of high society and can clean up very well when truly needed (although she will likely complain all the way).

As long as it remain exceptionnal, I would let a player go against their OPH, with perhaps one will roll at the start, and another to avoid reverting to their usual self mid-performance, unless they roleplay a temporary disadvantage (bad temper, or another OPH as suggested by khorboth) in which case I would probably drop the will rolls.

It is an habit, not a feature set in stone. The disads description, "You usually ..." certainly allow flexibility.

If they can switch it on/off at will, it become a quirk and will need to be bought off.