Consider the motion of a viscous incompressible fluid filling a 3D exterior domain \(\Omega \) subject to the Navier slip-with-friction boundary condition as well as outflow at infinity. For the Oseen system as the linearization, we discuss the resolvent set under a certain relationship among the geometry of the boundary \(\partial \Omega \) , friction coefficient \(\alpha (x)\) and the outflow \(u_\infty \) . We then study the regularity of the resolvent near the origin in the complex plane to develop \(L^q\) - \(L^r\) decay estimates of the Oseen semigroup provided that \(\alpha (x)+u_\infty \cdot \nu (x)/2\ge 0\) for every \(x\in \partial \Omega \) , where \(\nu (x)\) stands for the outward unit normal to the boundary \(\partial \Omega \) .