  Origin of wide varieties of quasi-periodic oscillation (QPO) observed in
compact sources is still not well established. Its frequencies range from mHz
to kHz spanning all compact objects. Are different QPOs, with different
frequencies, originating from different Physics? We propose that the emergence
of QPOs is the result of nonlinear resonance of fundamental modes present in
accretion disks forced by external modes including that of the spin of the
underlying compact object. Depending on the properties of accreting flow, e.g.
its velocity and gradient, resonances, and any mode locking, take place at
different frequencies, exhibiting low to high frequency QPOs. We explicitly
demonstrate the origin of higher frequency QPOs for black holes and neutron
stars by a unified model and outline how the same physics could be responsible
to produce lower frequency QPOs. The model also predicts the spin of black
holes, and constrains the radius of neutron stars and the mass of both.
