Another way to convert your .ovf files to .vts/vtk format is via mumax3-convert command
i.e. to convert your ovf to vts and get information about the whole magnetization vector use
mumax3-convert -vtk ascii *.ovf
If you want to get information about specific magnetization component use the following command
mumax3-convert -vtk ascii -comp ? *.ovf
? getting values 0 for x component 1 for y component and 2 for z component.
After that use an external software like paraview/mayavi/Visit to visualise your vts files.
The above-mentioned working for Linux OS. I am not sure about Windows