According to Osho, after Mahaparinirvana—the seventh-body ‘Great Nirvana’—a Buddha cannot personally return. Yet Buddha’s vow to appear as Maitreya is fulfilled by a different route: before dissolving, he intentionally left his etheric, astral, and mental bodies suspended as energy. When a fit being arises, these three bodies enter (parakaya-pravesh), recreating Buddha’s presence and wisdom as Maitreya without reversing Mahaparinirvana.
Buddha didn’t come back after final enlightenment; he left subtle parts that can enter a worthy person later, showing up as Maitreya.