Last week, O’Neil filed nominating petitions with the city, but had not officially announced his bid.
“I worked tirelessly for victims of sexual assault, for gun violence victims. I know what needs to be done to reduce our gun violence problem,” O’Neil said in a statement.
O’Neil was a prosecutor in the Special Victims Unit before he left the office in 2016 to enter private practice.
Teresa Carr Deni, Tariq El-Shabazz, Joe Khan, Lawrence Krasner, Rich Negrin, and Michael Untermeyer also filed nominating petitions with the city.