  We derive an asymptotic formula for operator product expansion coefficients
of heavy operators in two dimensional conformal field theory. This follows from
modular invariance of the genus two partition function, and generalises the
asymptotic formula for the density of states from torus modular invariance. The
resulting formula is universal, depending only on the central charge, but
involves the asymptotic behaviour of genus two conformal blocks. We use
monodromy techniques to compute the asymptotics of the relevant blocks at large
central charge to determine the behaviour explicitly.
