A2BO4 spinels constitute one of the largest groups of oxides, with potential applications in many areas of technology, including (transparent) conducting layers in solar cells. However, the electrical properties of most spinel oxides remain unknown and poorly controlled. Indeed, a significant bottleneck hindering widespread use of spinels as advanced electronic materials is the lack of understanding of the key defects rendering them as p-type or n-type conductors. By applying first-principles defect calculations to a large number of spinel oxides the major trends controlling their dopability are uncovered. Anti-site defects are the main source of electrical conductivity in these compounds. The trends in anti-sites transition levels are systemized, revealing fundamental doping rules, so as to guide practical doping of these oxides. Four distinct doping types (DTs) emerge from a high-throughput screening of a large number of spinel oxides: i) donor above acceptor, both are in the gap, i.e., both are electrically active and compensated (DT-1), ii) acceptor above donor, and only acceptor is in the gap, i.e., only acceptor is electrically active (DT-2), iii) acceptor above donor, and only donor is in the gap, i.e., only donor is electrically active (DT3), and iv) acceptor above donor in the gap, i.e., both donor and acceptor are electrically active, but not compensated (DT-4). Donors and acceptors in DT-1 materials compensate each other to a varying degree, and external doping is limited due to Fermi level pinning. Acceptors in DT-2 and donors in DT-3 are uncompensated and may ionize and create holes or electrons, and external doping can further enhance their concentration. Donor and acceptor in DT-4 materials do not compensate each other, and when the net concentration of carriers is small due to deep levels, it can be enhanced by external doping.