We study the long-time behaviour of helically symmetric infinite-energy solutions to the incompressible Navier-Stokes equations in the whole space R^3. Our solutions are H^1-perturbations of a Lamb-Oseen vortex whose circulation Reynolds number can be any fixed real number. If v denotes the helical velocity perturbation, no matter how large at initial time in H^1(R^3), we show that the scale-invariant quantities ||v(t)||_L^2 and √ t ||∇v(t)||_L^2 converge to zero as t → +∞. This proves that the Oseen vortex is globally stable with respect to H^1-helical perturbations. Our analysis relies on a logarithmic energy estimate for the perturbation v, on the Ladyzhenskaya inequality for helical vector fields, and on Poincaré's inequality which implies an exponential decay in time for the velocity components whose mean value is zero along the symmetry axis.