The Jet Propulsion Lab's 'Mars Global Surveyor' web site says thatOM "is the largest of the major Tharsis volcanoes rising...25 kilometers" from its base. The Johnson Space Flight Center at their OM page says "This shield volcano measures 25 kilometers" from its base. The textbook "Discovering Astronomy" by Robbins, Jeffrys and Shawl say that it rises nearly 27 kilometers from the surface of Mars. The textbook "Discovering the Universe" by William Kaufmann and Neil Comins says that it rises 24 kilometers above the surrounding plains. Some of these may simply be estimates while others may be attempts at being more accurate.

The volcano seems to have a roughly 2 kilometer escarpment above the local surface of Mars, and if the edge of this escarpment is being identified as the 'base' of the shield, then the JPL and JSFC estimates make sense, while the Robbins et al estimate of 27 kilometers can also be accommodated.