ob-4 +
(Latin: down)
Before c, ob- becomes oc-; before f, ob- becomes of-; before g, ob- becomes og-; before p, ob- becomes op-; before m, ob- becomes o-
occasion
1. A particular time; especially, as marked by certain circumstances or occurrences: "They met on three occasions."
2. A special or important time, event, ceremony, celebration, etc.: "His birthday will be quite an occasion."
3. A convenient or favorable time, opportunity, or juncture: "This slack period would be a good occasion to take an inventory."
4. The immediate or incidental cause or reason for some action or result: "What is the occasion for this noise?"