O-core is a new type transformer core after EI core, C-core, toroidal core and R-core. It is wound from grain oriented silicon steels with low iron loss and high flux density, which is slit into curve strips by computer-controlled slitting machine. The external shape of O-core is round or oval, which is similar to that of toroidal core, but its cross-section is round which is similar to that of R-core. Therefore, O-core combines the advantages of both toroidal core and R-core.

The round cross-section improves the electric performance of the core. The tight windings and the fact of no shape angles provide a well-distributed magnetic field. The improved coupling reduces magnetic leakage. The impedance is reduced accordingly as the length of each turn is reduced. The overload capacity improved consequently. Similar to R-core, O-core is wound from one continuous silicon steel strip and there are no joints, so the problem of humming can be solved radically.