Various ways developed so far in modeling oscillating/pulsating heat pipes (OHPs/PHPs) are briefly reviewed to find which way would be serviceable for design purposes and also be helpful to mathematically plainly describe oscillatory/circulatory motions of the charged working fluid. A selected way basically follows Ma's approach but a theoretically new attempt is made to derive the oscillation angular frequency ratio from two differently represented expressions of the oscillation velocity. A two-phase flow and evaporative/condensing heat transfer analysis is then carried out to get the wave equation of pressure oscillation. Finally obtained are closed-form algebraic expressions, providing us with convenient means of predicting the oscillation frequency- and-amplitude and the wave velocity. To demonstrate the applicability of those expressions, numerical comparisons are extensively done between our predictions and many other ones.