K0- and B0- oscillations are highly sensitive probes of New Physics (NP), since they only occur at loop-level within the Standard Model (SM). To be able to extract high-precision NP effects from experiment it is necessary to know SM predictions for these processes, which requires the calculation of NNLO QCD corrections. The short-distance Wilson coefficient Ctt is the dominant contribution to K0 oscillations, and the only (to a high precision) contribution to B0 oscillations, but has only been calculated to NLO, compared to other less important Wilson coefficients. In this talk I shall discuss the steps necessary for calculating Ctt, focussing on the resultant 3-loop integrals and their asymptotic expansion.