Is All-Is-One a buddhist belief?

I say - as I say to many I speak with - "abandon the idea of oneness, my friends, or else it will carry you off in the same way a lion carries off the lifeless carcass of an impala." Some listen, but some don't! Lol

Having said that, there is what one might call a 'region' along the way where the sense of an all encompassing oneness captivates you, but if you notice, very carefully, there are very tenuous fragments of an objectified 'self' claiming the oneness. In the Theravada tradition, this usually occurs around the six and seventh fetters, and is often called the 'I AM'. It may have been what Ven. Khemaka was struggling with when he said, "Friends, even though a noble disciple has abandoned the five lower fetters, he still has with regard to the five clinging-aggregates a lingering residual 'I am' conceit, an 'I am' desire, an 'I am' obsession." It is the case that, in some traditions, this is seen as somewhat of an endpoint, but, no! it is not! Ven. Khemaka was clearly on the ball!

The mind can make objects of the world appear luminous, bright and enchanting, and borders between the body and the outside world begin to fall away. In this state of oneness, the sense of self falls under its spell. One may walk around for decades pretending they are enlightened, trying to hanker onto that very luminosity and all the other interesting things that occur in that region. Fools!

"But, lord, might there be agitation over what is internally not present?"

"There might, monk," the Blessed One said. "There is the case where someone has this view: 'This cosmos is the self. After death I will be constant, permanent, eternal, not subject to change. I will stay just like that for an eternity.' He hears a Tathagata or a Tathagata's disciple teaching the Dhamma for the elimination of all view-positions, determinations, biases, inclinations, & obsessions; for the stilling of all fabrications; for the relinquishing of all acquisitions; the ending of craving; dispassion; cessation; Unbinding. The thought occurs to him, 'So it might be that I will be annihilated! So it might be that I will perish! So it might be that I will not exist!' He grieves & is tormented, weeps, beats his breast, & grows delirious. It's thus that there is agitation over what is internally not present."

"But, lord, might there be non-agitation over what is internally not present?"

"There might, monk," the Blessed One said. "There is the case where someone doesn't have this view: 'This cosmos is the self. After death this I will be constant, permanent, eternal, not subject to change. I will stay just like that for an eternity.' He hears a Tathagata or a Tathagata's disciple teaching the Dhamma for the elimination of all view-positions, determinations, biases, inclinations, & obsessions; for the stilling of all fabrications; for the relinquishing of all acquisitions; the ending of craving; dispassion; cessation; Unbinding. The thought doesn't occur to him, 'So it might be that I will be annihilated! So it might be that I will perish! So it might be that I will not exist!' He doesn't grieve, isn't tormented, doesn't weep, beat his breast, or grow delirious. It's thus that there is non-agitation over what is internally not present."

Abandoning Possessions & Views "Monks, you would do well to possess that possession, the possession of which would be constant, permanent, eternal, not subject to change, that would stay just like that for an eternity. But do you see that possession, the possession of which would be constant, permanent, eternal, not subject to change, that would stay just like that for an eternity?"

"No, lord."

"Very good, monks. I, too, do not envision a possession, the possession of which would be constant, permanent, eternal, not subject to change, that would stay just like that for an eternity.

"Monks, you would do well to cling to that clinging to a doctrine of self, clinging to which there would not arise sorrow, lamentation, pain, grief, & despair. But do you see a clinging to a doctrine of self, clinging to which there would not arise sorrow, lamentation, pain, grief, & despair?"

"No, lord."

"Very good, monks. I, too, do not envision a clinging to a doctrine of self, clinging to which there would not arise sorrow, lamentation, pain, grief, & despair.

"Monks, you would do well to depend on a view-dependency (ditthi-nissaya), depending on which there would not arise sorrow, lamentation, pain, grief, & despair. But do you see a view-dependency, depending on which there would not arise sorrow, lamentation, pain, grief, & despair?"

"No, lord."

"Very good, monks. I, too, do not envision a view-dependency, depending on which there would not arise sorrow, lamentation, pain, grief, & despair.

"Monks, where there is a self, would there be [the thought,] 'belonging to my self'?"

"Yes, lord."

"Or, monks, where there is what belongs to self, would there be [the thought,] 'my self'?"

"Yes, lord."

"Monks, where a self or what belongs to self are not pinned down as a truth or reality, then the view-position — 'This cosmos is the self. After death this I will be constant, permanent, eternal, not subject to change. I will stay just like that for an eternity' — Isn't it utterly & completely a fool's teaching?"

"What else could it be, lord? It's utterly & completely a fool's teaching."

Alagaddupama Sutta

Khemeka Sutta