To assess the geometry of cracks in highly altered peridotites, we analyzed the ultrasonic velocity of serpentinized dunites and harzburgites collected by the Oman Drilling Project (Holes BA1B, 3A, and 4A). First, we estimated the hydrated fraction from grain density to obtain the porosity-free matrix velocity, which indicated complete serpentinization at shallow depths and decreasing hydration at greater depths. We assume that the difference between the solid matrix and measured onboard ultrasonic velocity is attributed to cracks with a spheroidal shape in the samples. Application of the effective medium theory to onboard data, such as P-wave velocity and porosity, indicates that the average pore aspect ratio is mostly between 0.1 and 0.01, and crack density varies from 0.58 to 0.02. We found a positive relationship between aspect ratio and hydrated fraction, suggesting a change in crack shape related to dissolution-precipitation processes during hydration. The relatively high aspect ratio and hence high fluid flux at shallow depths are also consistent with the onboard resistivity data and present-day hydration processes inferred from the borehole fluid chemistry. The inversion of ultrasonic data provides a series of elastic moduli that can be used to make a rough approximation of Poisson's ratio from the onboard data, which is a key physical property for interpreting geophysical observations in the oceanic lithosphere.