We consider systems of quasi-periodic linear differential equations associated to a ‘resonant’ frequency vector
${\it\omega}$, namely, a vector whose coordinates are not linearly independent over
$\mathbb{Z}$. We give sufficient conditions that ensure that a small analytic perturbation of a constant system is analytically conjugate to a ‘resonant cocycle’. We also apply our results to the non-resonant case: we obtain sufficient conditions for reducibility.