Any difficulty level will never have an Ohm rune dropped by either The Countess or the Hellforge, no matter what the Diablo 2 resurrected Runewords may be. It can either drop randomly on Act 2 or later on Hell difficulty, or it can be created by combining two Vex runes and a normal emerald in the Horadric Cube on Act 2 or later on Hell difficulty. It can also be obtained by combining two Vex runes and a normal emerald in the Horadric Cube on Hell difficulty. It can also be obtained by combining two Vex runes and a normal emerald in the Horadric Cube on Hell difficulty, which is a rare combination.