Dr. King uses the words "heat of injustice" and "heat of oppression" to describe Mississippi because

Metaphor: King compares injustice and oppression to sweltering heat and freedom and justice to an oasis.

King repeats the sweltering heat metaphor toward the end of the speech, referring specifically to Mississippi, a state where some of the worst offenses against blacks had been carried out. By specifying states in the south (he also mentions Georgia, Tennessee, Alabama, and the South in general) and mentioning the oasis that awaits even these places, King magnifies his message of hope to those suffering the most.