I probably should have clarified that the professor was himself a lay person. Basically the term "witch", "saint" or "heretic" was applied based on the social context of the individual, or a moral judgment made by the church or the population surrounding the individual. Heretics were people the Church didn't agree with, saints were those whose actions were approved by the Church. The Church has been known to change its stance over time, and some individuals have fallen into multiple categories at various times. Famously, Joan of Arc was at various points a witch, a heretic, and a saint.

Regarding Olga, she christianized Rus and here sainthood was recognition of that. I don't actually know if she was ever considered a saint by the Roman Church.