Omarosa -- Hey, Donald Trump ... Lay Off President Obama! You Don't Know JACK About Ferguson

Omarosa Manigault is ripping her old boss Donald Trump ... saying he's talking out of his ass when he blames President Obama for the Ferguson riots -- and Trump has no clue what's really happening there.

Omarosa -- a veteran of 3 seasons of "The Apprentice" -- tells TMZ Trump is entitled to his opinion ... but claims he's making up facts when he says Obama's weak leadership is responsible for America's racial divide.

Trump tweeted ... Obama has "no control (or respect) over the African-American community" -- and blamed him for blacks doing "so poorly" under his presidency.

Omarosa tells us ... she thinks Trump is full of BS, and claims she personally spent time in Ferguson working alongside White House personnel sent by the President to deal with the crisis.

She also says Trump is the one who's making things worse because -- "With tensions high in our community we don't need more divisiveness! And the Obama bashing is getting a bit redundant."

So much for that 4th go-round on 'Apprentice.'