We demonstrate via first-principles calculations a novel method of tuning the electron-electron interactions in bulk oxide materials via controlling the cationic layer arrangement. Using the Ruddlesden-Popper oxides LaSrMnO4 and LaSrTiO4 as examples, our study demonstrates that a self-doping effect can be induced by changing the stacking of the neutral and charged cationic layers. It is believed that such a phenomenon is associated with different movements of apical oxygen atoms, resulting in diverse bandgaps, magnetism and orbital degrees of freedom in the same stoichiometric strongly-correlated material. This finding may open up a new direction to engineer the transition metal oxides for practical applications requiring tunable electronic properties without external doping.