The 18-year-old centre-half, who has played for England at Under-18 and Under-19 level, will stay with the Minstermen for a month.
O'Connell, who made five appearances for Rotherham while on loan there at the back end of last year, will go straight into York's squad for Saturday's league game at Gillingham.
York have also been linked with another teenage defender, Jameel Ible, who plays for non-league Salford City, while former Cheltenham and Barnsley winger Martin Devaney is currently on trial at the club.
