In this communication a simple technique based on the phasor's method is presented in order to analytically deal with nearly sinusoidal oscillator constraints. Unlike other methods, more complex from a computational point of view, such as Volterra series and Poincare's perturbation, we are able to set the main linearity performance of a weakly nonlinear oscillator through closed form relationships. This gives to the designer a deep insight into the circuit behavior. In particular, we applied the methodology to the case of the Hartley oscillator, finding the expression of amplitude and frequency of the fundamental, and second- and third-order harmonic distortion factors as functions of the circuit components. Simulations performed with Spectre are provided and the results are found in good agreement with those expected.