We report optical ab-plane properties of the layered sodium cobaltate, Na_xCoO₂ for x = 0.35 and 0.75. Two samples show metallic behaviors according to dc resistivity transport. Overall temperature dependent optical conductivities of both samples are very similar to those of the high temperature superconducting underdoped cuprates. We found that the optical scattering rate of x = 0.75 sample, which is in a Curie-Weiss metallic phase, varies linearly (non-Fermi liquid) with frequency and temperature while that of x = 0.35 sample, which is in a paramagnetic metallic phase, varies quadratically (Fermi liquid) with frequency and temperature. Both x = 0.35 and 0.75 samples have an onset of scattering around 600 cm^-1 which can be attributed to the interaction of charge carriers with a bosonic collective mode in a system.