Here you can find straight-to-the-point tutorials on how to start using the Witnet oracle and other relevant topics like operating Witnet nodes and mining Wit tokens.
Data Feeds Tutorial
The Data Feeds tutorial gives a quick overlook on what are data feeds, how the Witnet data feeds work, and how to start using them with only a few lines of Solidity:
The Randomness tutorial introduces the concept of randomness sources, how the Witnet randomness source work, and how to start generating random numbers with only a few lines of Solidity:
The APIs and HTTP GET/POST tutorial is a quick introduction to how totally custom data requests can be built with Witnet to use any arbitrary HTTP(S) API as a data source for your smart contracts: