  We apply the $Om$ diagnostic to models for dark energy based on scalar
fields. In case of the power law potentials, we demonstrate the possibility of
slowing down the expansion of the Universe around the present epoch for a
specific range in the parameter space. For these models, we also examine the
issues concerning the age of Universe. We use the $Om$ diagnostic to
distinguish the $\Lambda$CDM model from non minimally coupled scalar field,
phantom field and generic quintessence models. Our study shows that the $Om$
has zero, positive and negative curvatures for $\Lambda$CDM, phantom and
quintessence models respectively. We use an integrated data base
(SN+Hubble+BAO+CMB) for bservational analysis and demonstrate that $Om$ is a
useful diagnostic to apply to observational data.
