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Oracles support the access, validation, and transmission of data from external sources to blockchain systems. They are important components of blockchain-based architectures. However, there exists no guidance on how oracles could be used when designing blockchain-based applications. In this paper, based on the results of a systematic literature review, we propose a framework to explain blockchain oracles and their relationships to blockchain-based applications. More specifically, the blockchain oracle framework addresses the origin of data, oracle properties, encryption method, oracle data source, validation procedures, and the integration of oracles to blockchain-based applications. Potentially, this framework can guide developers when incorporating oracles to blockchain-based applications.