Once a contract is deployed on the blockchain, its source code is immutable. So before using a contract, check if it gives permission to its deployer (or any address) to change any state at will.

Note that some contracts act as proxy to other contract and can be made to point to another code through a state change, if this is the case then you need to trust whoever can change the state to point to another contract. Such contract sometime have a timelock so that if such a change occurs, there's a delay before it is actually activated, which gives time to users to withdraw their funds if they do not trust the update.

If you are talking about Oracle contracts, if it's an oracle involving offchain data, then there will always be some trust involved, which is usually managed by having the offchain actors share the responsibility and staking some money with the risk to get slashed if they turn into bad actors. But again, offchain data oracles will always require some level of trust that would have to deal with in non-blockchain apps too.