Blockchains and smart contracts can’t access off-chain data (outside the network). However, for many contractual arrangements, it is vital to have relevant data from the outside world to execute the agreement.
Blockchain Oracles are entities that connect blockchains to external systems, thereby enabling smart contracts to execute based on inputs and outputs from the real world. Simply put, Oracles provide a mechanism for the decentralized Web3 ecosystem to access the outside world.
KCC Oracle: KCC Oracle, a decentralized Web3 Oracle, is a data feed network that connects real-world data to blockchain smart contracts.
Pyth: Pyth delivers real-time on-chain market data. Refer here for API Reference.
Witnet: Witnet enables your smart contracts to react to real-world events with strong crypto-economic guarantees. Refer here for API Reference.