It's theoretically possible to HIO on levels 1-5, My 5:54 HIO'ed on levels 3 and 4.

1: HIO (Theoretically, a frame perfect hit could send you far enough, and with a good angle, a possible HIO.) 2: HIO 3: HIO 4: HIO 5: HIO (Unsure)

2,3,4,5 hio have already been done, 1 hio has already been confirmed impossible

i have an unrecorded 3:27. maybe someday ill record a run of that level.

is there any forum thread of 1 hio being confirmed impossible?

No. I usually chat with some pros in a discord server about tricks. That's how i keep doing new and exciting trickshots in runs. They got close, but ultimately could not achieve oasis 1 hio. Achieving oasis 1 hio would require a glitch beyond the Birdie strategy.