if you're going to use osb, keep the noise frequency and material mu in mind. For wood that's about 700Hz for 1". Use an app like Spectroid to determine the frequency of your rain noise. If it's 1400Hz, then a half inch will work, 2800Hz=1/4", whereas if it's 350Hz, then you'll need 2" to absorb the wavelength. You'll get hardly any Db attenuation from 1/2 wavelength thickness, and a linear-like reduction from 1/2Mu to 1Mu, where it will be mostly blocked.