The structure of exotic nuclei provides valuable tests for state-of-the-art nuclear theory. In particular electromagnetic transition rates are more sensitive to aspects of nuclear forces and many-body physics than excitation energies alone. We report the first lifetime measurement of excited states in $^{21}$O, finding ${\tau }_{1/2^{+}}={420}_{-32}^{+35}{\text{(stat)}}_{-12}^{+34}\text{(sys)}$ ps. This result together with the deduced level scheme and branching ratio of several γ-ray decays are compared to both phenomenological shell-model and ab initio calculations based on two- and three-nucleon forces derived from chiral effective field theory. We find that the electric quadrupole reduced transition probability of $B(E2;1/2^{+}\to 5/2_{g.s.}^{+})={0.71}_{-0.06-0.06}^{+0.07+0.02}$ e$^{2}$fm$^{4}$, derived from the lifetime of the $1/2^{+}$ state, is smaller than the phenomenological result where standard effective charges are employed, suggesting the need for modifications of the latter in neutron-rich oxygen isotopes. We compare this result to both large-space and valence-space ab initio calculations, and by using multiple input interactions we explore the sensitivity of this observable to underlying details of nuclear forces.