  We derive a nonperturbative, convergent operator product expansion (OPE) for
null-integrated operators on the same null plane in a CFT. The objects
appearing in the expansion are light-ray operators, whose matrix elements can
be computed by the generalized Lorentzian inversion formula. For example, a
product of average null energy (ANEC) operators has an expansion in the
light-ray operators that appear in the stress-tensor OPE. An important
application is to collider event shapes. The light-ray OPE gives a
nonperturbative expansion for event shapes in special functions that we call
celestial blocks. As an example, we apply the celestial block expansion to
energy-energy correlators in N=4 Super Yang-Mills theory. Using known OPE data,
we find perfect agreement with previous results both at weak and strong
coupling, and make new predictions at weak coupling through 4 loops (NNNLO).
