Oral interpreters aren't really very common anymore (if they ever were) - mainly because systems like CART etc are much more common, reliable and it is generally easier to train a CART operator than an Oral Interpreter.
For those who aren't aware of CART it is a system by which speech is converted by a transcriptionist/typist into "real-time" text which can be read privately on an individual's computer screen (if there is only a single CART user) or displayed on a large screen (for multiple CART users). The added "bonus" of using CART for educational settings is that the entire "word-for-word" text transcription can be saved to a computer file and given to the CART user (generally a deaf/Hoh person) and used much the same way that hearing students may elect to using an audio recording device in class to record the entire spoken content of the class so that they can listen to it again at a future date
Hope that helps !