  Baryon Acoustic Oscillations (BAO) provide an important standard ruler which
can be used to probe the recent expansion history of our universe. We show how
a simple extension of the Om diagnostic, which we call Om3, can combine
standard ruler information from BAO with standard candle information from type
Ia supernovae (SNIa) to yield a powerful novel null diagnostic of the
cosmological constant hypothesis. A unique feature of Om3 is that it requires
minimal cosmological assumptions since its determination does not rely upon
prior knowledge of either the current value of the matter density and the
Hubble constant, or the distance to the last scattering surface. Observational
uncertainties in these quantities therefore do not affect the reconstruction of
Om3. We reconstruct Om3 using the Union 2.1 SNIa data set and BAO data from
SDSS, WiggleZ and 6dFGS. Our results are consistent with dark energy being the
cosmological constant. We show how Om and Om3 can be used to obtain accurate
model independent constraints on the properties of dark energy from future data
sets such as BigBOSS.
