The invention relates to a method of operating a wind turbine (1) to guard against oscillations of the wind turbine blades (5) when they are at a standstill or idling at low speeds, and to a wind turbine blade anti-oscillation apparatus (10). The apparatus comprises a releasably attachable blade cover (10) that provides a non-aerodynamic surface for a region of the blade. This has been found to prevent air flow adhering to the blade and periodically detaching in a phenomenon known as vortex shedding, and therefore prevents oscillations of the blade becoming problematic. The blade cover can comprise a sleeve (10) of a net-like material, that can be positioned on the blade either before installation or in the field by service engineers using guide lines 16 and 17.