Ordered magnetism in the intrinsically decorated <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>j</mml:mi><mml:mi>eff</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mfrac><mml:mn>1</mml:mn><mml:mn>2</mml:mn></mml:mfrac><mml:mspace width="4pt"/><mml:mi>α</mml:mi><mml:mtext>−</mml:mtext><mml:msub><mml:mi mathvariant="normal">CoV</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>8</mml:mn></mml:msub></mml:math>

The antiferromagnetic mixed valence ternary oxide $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CoV}}_{3}{\mathrm{O}}_{8}$ displays disorder on the ${\mathrm{Co}}^{2+}$ site that is inherent to the $Ibam$ space group resulting in a local selection rule requiring that one ${\mathrm{Co}}^{2+}$ and one ${\mathrm{V}}^{4+}$ reside next to each other, thus giving rise to an intrinsically disordered magnet without the need for external influences such as chemical dopants or porous media. The zero-field structural and dynamic properties of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CoV}}_{3}{\mathrm{O}}_{8}$ have been investigated using a combination of neutron and x-ray diffraction, dc susceptibility, and neutron spectroscopy. The low-temperature magnetic and structural properties are consistent with a random macroscopic distribution of ${\mathrm{Co}}^{2+}$ over the $16k$ metal sites. However, by applying the sum rules of neutron scattering we observe that the collective magnetic excitations are parametrized with an ordered ${\mathrm{Co}}^{2+}$ arrangement and critical scattering consistent with a three-dimensional Ising universality class. The low-energy spectrum is well described by ${\mathrm{Co}}^{2+}$ cations coupled via a three-dimensional network composed of competing ferromagnetic and stronger antiferromagnetic superexchange within the $ab$ plane and along $c$, respectively. While the extrapolated Weiss temperature is near zero, the 3D dimensionality results in long-range antiferromagnetic order at $T{}_{\mathrm{N}}\ensuremath{\sim}19\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. A crystal field analysis finds two bands of excitations separated in energy at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\sim}5\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ and 25 meV, consistent with a $j{}_{\mathrm{eff}}=\frac{1}{2}$ ground state with little mixing between spin-orbit split levels. A comparison of our results to the random 3D Ising magnets and other compounds where spin-orbit coupling is present indicate that the presence of an orbital degree of freedom, in combination with strong crystal field effects and well-separated $j{}_{\mathrm{eff}}$ manifolds, may play a key role in making the dynamics largely insensitive to disorder.