In a general sense, oracles are "portals"/"vessels" through which formless beings can enter and speak directly to people. They provide the "message from the Gods" - and in Vajrayana Buddhism, they are like a speaking divination box.
Oracles are different from seers, in that they themselves become the "Gods", not interpret signs by the Gods through tea leaves signs, animal entrails signs etc.
In Buddhism, the most "famous" is the Nechung Oracle which is the State Oracle of Tibet. There are also different oracles who can take trance of enlightened protectors such as Setrap and Dorje Shugden. Then there are also different oracles who can take trance of unenlightened protectors such as Jowo Chingkar, local deities etc.
My question is, if there are oracles which can take trance of enlightened protectors, are there oracles which can take trance of enlightened Buddhas? If no, why not?