The UFC honcho is furious with the boxing legend after Oscar publicly trashed the Mayweather vs. McGregor negotiations ... calling it a "joke of a fight" that could kill the sport of boxing.

Earlier Wednesday, White posted a video on Instagram showing Oscar praising a UFC vs. Boxing fight four months ago ... and then trashing it now.

White says Oscar is coming off as an insecure hypocritical weirdo for changing his position so drastically -- and says it makes it sound like Oscar has no confidence in the boxing match he's promoting, Canelo Alvarez vs. Gennady Golovkin.

You gotta watch the clip ... Dana goes absolutely nuclear -- plus, he gives the latest on the Floyd vs. Conor negotiations.