We have studied the electronic and magnetic properties of the doped quaternary compounds Os1−x−y MnxCryAl2(x + y ≤ 0.5) in MoSi2 type structure with varying composition ratio of Mn to Cr by first-principles calculations. Results show that Os1−x−y MnxCryAl2 have high spin-polarizations at the Fermi level with the Mn and Cr concentration in the range of x + y ≤ 0.5(y ≠ 0). The magnetic properties of the Os1−x−y MnxCryAl2 are robust against the site configurations of Os, Mn, and Cr atoms. The total spin magnetic moments of the quaternary compounds with high spin polarizations at the Fermi level obey the Slater–Pauling rule in the form of Mt = 28 − Nt approximately.