We consider mild solutions to the 3D time-dependent Oseen system with homogeneous Dirichlet boundary conditions, under weak assumptions on the data. Such solutions are defined via the semigroup generated by the Oseen operator in L^q. They turn out to be also L^q-weak solutions to the Oseen system. On the basis of known results about spatial asymptotics of the latter type of solutions, we then derive point-wise estimates of the spatial decay of mild solutions. The rate of decay depends in particular on L^p-integrability in time of the external force.