"ECONOMY_OVERLORD" seemed to happen at the end of the month. Not always, but they were likely highly to.
Other than that, other types of OOS could happen any day, some times even several days in row.
Completed the run when game year reached 2600, wanted to experience all end-game crisis events. Looking at the OOS-folder, there is 45 folders, totaling about 19 GB of files.
Edit: Overall list of OOS I can remember from the game:
- "NUM_DEPOSITS"
- "LAST_EVENT_ID"
- "SHIP_DESIGN"
- "NUM_POP_GROUPS"
- "ECONOMY"
- "ECONOMY_OVERLORD"
- "NUM_MISSILES"
- "RANDOM_COUNT"
RANDOM_COUNT seemed to mostly happen if you decided to ignore the previous OOS by either leaving the re-sync window on screen, or by re-syncing using that. Only effective method to delay it was by re-hosting from main menu.