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The electronic structure, magnetism and Fermi surface of a hole self-doped potential iron-based superconductor KCa 2 Fe 4 As 4 O 2 (12442) were studied by a first-principles DFT/GGA approach. The self-doping value is obtained from DFT is 0.75 hole on Fe-3d states for KCa 2 Fe 4 As 4 O 2. Surprisingly, for the KCa 2 Fe 4 As 4 O 2 it was found that except the Fe-3d bands the O-2p bands also cross the Fermi level, which is quite anomalous behavior for O-2p states. In general, the O-2p states are usually fully occupied in transition metal oxides. Here it is not the case due to the fact that CaO layer gives some of the electrons to the FeAs layer, thus leaving O-2p states partially occupied. The O-2p states form additional Fermi surface sheet with propeller-like shape around Γ -point and modify the Fe-3d Fermi surface sheets due to the hybridization. To prove the presence of anomalous metallic O-2p band in KCa 2 Fe 4 As 4 O 2 the comparative study with the same family systems KCa 2 Fe 4 As 4 F 2 and RbGd 2 Fe 4 As 4 O 2 (where it is not the case) was performed. Finally, we discuss the main differences between the emergence of the metallic O-2p band in case of KCa 2 Fe 4 As 4 O 2 and those in case of cuprate superconductors LaBaCuO 4 and YBa 2 Cu 3 O 7. Therefore, we expect that the KCa 2 Fe 4 As 4 O 2 might be a potential superconductor with unusual properties. [ABSTRACT FROM AUTHOR]