According to Osho, Krishna is called a complete incarnation because he is ‘zero’—utterly empty, choiceless, without any fixed somebody. Like the indivisible zero from which all numbers arise and to which they return, his zero‑personality contains everything. Refusing definition, he reveals rather than describes himself; being nobody, he can be everybody, acting spontaneously without bondage—hence purnavatara.
Krishna is complete because he became like zero—nothing to cling to or call himself—so he could be everything and respond freely.