Original ATK and DEF (元々の攻撃力(守備力) Motomoto no Kōgekiryoku (Shubiryoku)) refers to the ATK and DEF values of a Monster Card before any monster, Spell, or Trap Cards alter the value. They are the ATK and DEF values listed on the card.
Certain monsters original ATK and DEF are determined only when the monster is Summoned to the field; when cards like "Chimeratech Overdragon" are Summoned, its original ATK and DEF is dependent on what was used to Summon it. Cards whose ATK and DEF are determined by these factors usually have a written ATK and/or defense of ? or 0. Additionally, if these monsters are banished from the field, and re-summoned, or flipped face-down and back up again, this resets their ATK and DEF, thus, their ATK and DEF would then be treated as 0, unless stated otherwise in said card's effect. Note that Monsters with a ATK and DEF of 0 would always be treated as having 0 ATK for the purposes of card effects, such as for Summoning through searchers or when their original ATK and DEF come into question (such as for the effect of "Megamorph"). Monsters with a ? for their ATK or DEF value cannot be searched through searchers, as they effectively have no ATK or DEF to measure before they are Summoned. However in most cases, even after they're Summoned, they are treated as having 0 original ATK and DEF.