An ontological change means a change of the very essence of a being. Since baptism permanently configures us to Christ, yes, it brings an ontological change. This is why the Church insists there can be only one baptism: once the ontological change takes place it cannot be repeated.
The Catechism of the Catholic Church:
1265 Baptism not only purifies from all sins but also makes the neophyte “a new creature,” an adopted son of God, who has become a “partaker of the divine nature,” member of Christ and co-heir with him, and a temple of the Holy Spirit.