How long before the meeting does a special meeting have to be posted?
The board needs to give 24 hours notice for a special meeting and "reasonable" notice for a regular meeting.
A regular meeting is one on the calendar adopted by the board at its first meeting after July 1. "Reasonable" is usually considered to be 24 hours, but it is defined more by "usual" local practice than anything; it isn't set out in statute.
Special meetings are meetings called throughout the year that aren't on the calendar of meetings established at the first board meeting of the year in July. ORS 192.640.