  We present the two-mass QCD contributions to the pure singlet operator matrix
element at three loop order in x-space. These terms are relevant for
calculating the structure function $F_2(x,Q^2)$ at $O(\alpha_s^3)$ as well as
for the matching relations in the variable flavor number scheme and the heavy
quark distribution functions at the same order. The result for the operator
matrix element is given in terms of generalized iterated integrals that include
square root letters in the alphabet, depending also on the mass ratio through
the main argument. Numerical results are presented.
