Phase noise measurements of an optoelectronic oscillator (OEO) at frequencies less than 10 Ha from the carrier (10.6 GHz) as well as the measured Allan variance are presented for the first time. The system has a measured single-side-band (SSB) phase-noise of -123 dB/Hz at 10 kHz from the carrier and a sigma(y)(tau)=10(-10) for an integration time between 1 and 10 seconds. The importance of amplifier phase-noise and environmental fluctuations in determining the noise of the oscillator at these low Fourier frequencies is verified experimentally and analyzed using a generalized model of noise sources in the OEOs. This analysis then allows prediction of the oscillator performance from measured parameters of individual components in the system.