Constrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund's exchange J parameters for Fe-3d electrons in LaFeAsO. Results strongly depend on the basis set. When O-2p, As-4p, and Fe-3d orbitals are included, computation results in U = 3-4 eV. With the basis set restricted to Fe-3d orbitals only, computation gives parameters corresponding to F0 = 0.8 eV, J = 0.5 eV. Local Density Approximation combined with Dynamical Mean-Field Theory calculation with these parameters results in weakly correlated electronic structure.