O-toku means 'a bargain' or 'a special'. The word toku is a Chinese-derived word that means 'benefit' or 'advantage'. The o- is prefixed as a form of respect by retailers towards the general public, informing them that this is to your 'honourable benefit', as the clichéd translation goes.
In Chinese characters, o-toku is written . This can be found frequently enough. :
But it's also very common to write toku in katakana as .
This usage is probably related to the aggressive advertising and promotional nature of the term. The character means simply 'to obtain', as a verb read eru. It is apt to be used i spoken sales pitches (okyakusama, o-toku desu yo! -- 'It's a bargain!'). Katakana are useful in emphasising this.
The following distribution of forms was found through a Google search in August 2003. is by far the favoured form.