| Summary: | The remarkable increase in the number of interconnected smart devices in today’s Internet of things networks introduces more challenges related to security, trust, and centralization, which require more effective solutions. Fortunately, blockchain technology has recently emerged as a potential rescuer for IoT-based solutions due to its decentralization and enhanced security features. It is usual for smart contracts to arise in handling and processing the generated data when IoT devices are combined with blockchain. However, blockchain and smart contracts need to interact with input data of the same level of trust to guarantee correct applications execution. This implies using oracles to provide trust compatibility between inserted information collected from IoT devices and blockchain and smart contracts. Therefore, this study adopts a methodology that was shaped based on current literature and design and experiments to provide a full narrative of the process of combining two of the most intriguing systems in today’s world of technology, namely, blockchain and IoT including a very important part of the comprehensive system, viz. blockchain oracle. Moreover, it was found that the literature lacks a complete view of the IoT-blockchain integration process that covers all its important and related aspects. Therefore, this work is an attempt to fill the gap in literature and contribute to the body of knowledge by surveying the literature about existing IoT-blockchain architectures and shed light on the role of blockchain in addressing IoT issues while demonstrating the concept of oracles as well as their functions in addition to the main operating blockchain oracles. Additionally, this work illustrates a CO<sub>2</sub> measuring use case where a smart contract is developed and tested as part of two proposed oracle-based designs. The obtained results demonstrate a full picture of a practical integrated IoT-blockchain system architecture.
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