Oliver Tree laid out the details of his will during an interview just weeks before his death ... and he says his family's not getting a dime.

The singer-songwriter -- who tragically passed away in a helicopter crash on Sunday -- appeared on the "Zach Sang Show" back in April ... and he told the host he doesn't feel the wealth he's made from music belongs to him.

Tree says he set up his will so most of his cash is going back to artists through a foundation he started.

The star notes in the clip that, historically speaking, artists make more after they die than they ever did alive ... and he set up the "Dr. Oliver Tree's Art Grants for Baby Geniuses" foundation to dole out any and all dough he made during his lifetime or that he rakes in after.

Tree says a committee of people he worked with while he was alive will vote on where the money should go -- and, if you listen to the clip, it sounds like all of this is already in place.

As you know ... Oliver and five others -- including YouTuber Gaspi -- died in Brazil after two helicopters collided near Rio de Janeiro.

The last video Oliver posted show his day in the Brazilian country ... full of fun, food and friends. Iae Break, an influencer who hung out with Oliver before his passing, shared a tribute post to him Sunday afternoon.

Oliver was 32.