Detectives interviewed the student victim about the allegations and determined the teen and teacher had indeed been engaged in a sexual relationship. In addition to physical abuse, O'Neill is accused of sending the girl explicit text messages with obscene material, authorities said.
Investigators say the crimes occurred at various locations within Barstow.
It was unclear whether the student was in any of O'Neill's classes.
The teacher was arrested Friday on suspicion of penetration with a foreign object, oral copulation with a minor, annoying and molesting and sending harmful matter to a minor.
O'Neill was released from custody the next day after posting $100,000 bail, officials said.
A court appearance has yet to be scheduled, inmate records show.
Detectives believe O'Neill could have additional victims.
Anyone with information can contact Deputy Gerania Navarro at 909-387-3615 or Detective Brian Grimm at 760-256-4838. Anonymous tips may be submitted via 800-782-7463.