In the ancient Greek religion, an oracle was generally a holy site at which a priest could ask the gods about the future. The most famous oracle was the Oracle at Delphi, where the god Apollo answered questions about the future. It is also common usage to talk about a person as an oracle.
Other pagans examined entrails and other signs to divine the future. This was more of a superstition, somewhat like reading tea-leaves today. This person could be called a diviner.