The kinematic molecular viscosity of air can be taken as 1.5e-5 m^2/s for a wide range of applications. You can refer to this table https://www.engineeringtoolbox.com/dry-air-properties-d_973.html.
Turbulent viscosity ratio = turbulent viscosity / molecular viscosity. For incompressible flow, OpenFOAM uses the kinematic version of viscosity. In this case you have kinematic viscosity ratio (nut/nu) = kinematic turbulent viscosity / kinematic molecular viscosity = kinematic turbulent viscosity / 1.5e-5.
The kinematic turbulent viscosity (nut) is calculated by the turbulence model. With each pair of k and omege values, you will get a unique nut.