All these are terms for someone able to bring a message from another reality to help us. They can often predict the future, challenge us about the past or provide spiritual guidance. Soothsayer means “truth sayer”. Seer is a more modern word indicating someone who can see for example in Crystal balls or water.
An Oracle is a more old-fashioned word dating back to the Ancient Greeks. It describes someone who through the use of drugs, dreams, or inhaling volcanic fumes was able to induce visions and inform people of the future. They are frequently associated with Oracular deities such as Apollo, who would give visions.
Very few people use these terms to refer to themselves in the modern day. They might also be met with scepticism since there are certain powers claimed.
See also other types of Magical Practitioner.