  We present an ab initio approach for the description of collective
excitations and transition strength distributions of arbitrary nuclei up into
the sd-shell that based on the No-Core Shell Model in combination with the
Lanczos strength-function method. Starting from two- and three-nucleon
interactions from chiral effective field theory, we investigate the electric
monopole, dipole, and quadrupole response of the even oxygen isotopes from 16-O
to 24-O. The method describes the full energy range from low-lying excitations
to the giant resonance region and beyond in a unified and consistent framework,
including a complete description of fragmentation and fine-structure. This
opens unique opportunities for understanding dynamic properties of nuclei from
first principles and to further constrain nuclear interactions. We demonstrate
the computational efficiency and the robust model-space convergence of our
approach and compare to established approximate methods, such as the Random
Phase Approximation, shedding new light on their deficiencies.
