Can You Start A Sentence With Or?
A word of caution: just because you can, it doesn't mean you should. Many English teachers get real puckered up when the one of their students starts a sentence with a conjunction, even though a lot of our language's best writers did it all the time.
Anyone encountering one of those teachers should request that the teacher unpucker and stop relying on simply buffaloing students. Request of that teacher that he or she locate accurate information on language and how it really works and provide such guidance to students.
That's exactly what a teacher should do.
But it might be advisable to go easy on this in formal writing.