If your distributed consensus determines its notaries and their weight long enough in advance, an oracle can announce them (if possible signed by current notaries) for use in by other consensuses. If you see the oracle lying, exit contracts at once, before the lie is effective.
Special case: if using Proof-of-Work, the oracle you need is being able to predict the difficulty long enough in advance for users to exit contracts if they see a lie in the published prediction.
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