abstract: In this talk we address the numerical solution of the quadratic optimal transport problem in its dynamical form, the so-called Benamou-Brenier formulation. When solved using interior point methods, the main computational bottleneck is the solution via iterative methods of large saddle point linear systems arising from the associated Newton-Raphson scheme. We will present a preconditioner based on the partial commutation of the operators in the dual Schur complement of these linear systems. A series of numerical tests show that this preconditioner is the most efficient among those considered, with a CPU-time scaling only slightly more than linearly with respect to the number of unknowns used to discretize the problem.