What is "OOO" in office English? Question

What is this?

Are all three letters 0 (zero), O (o), or even 0 (circle)?
It is hard to imagine that the English expression uses zeros (circles), so the only ones left are zeros or o's.

Hint
Although many people manage their schedules on PCs these days, this expression is not used for people in the office.

We will send out the answers at 12:00 on December 21 (Mon.), around the time everyone starts their lunch break, so please look forward to it!