Can a Druid start the game in the Oppidum in Roman occupied Corinium?
Risky but Yes. If no revolt, the Druid will be killed which is too early of a loss of a rebel rouser.
Can the Romans prevent Druids from entering Oppidums buy occupying them?
Yes, as per rule 7.1.13. The Tribes with the Druid would have to fight the occupying garrison. A Druid by himself cannot fight and would just be killed.