"The Organization too Secret to Know," or "O2STK" as the Middleman and Ida call it, is the group which supplies the fields orders to the Middleman. The only one in direct contact with the organization is Ida, and that only works one way. It is assumed that O2STK pays for everything at Middleman HQ.
The Organization was once known as "WTHWWF" or "Whoever the Heck We Work For." This was a joke between the 1969 Middleman and Ida.