mid-14c., oppressioun, "cruel or unjust use of power or authority," from Old French opression (12c.), from Latin oppressionem (nominative oppressio) "a pressing down; violence, oppression," noun of action from past-participle stem of opprimere "press against, press together, press down;" figuratively "crush, put down, subdue, prosecute relentlessly" (in Late Latin "to rape"), from assimilated form of ob"against" (see ob-) + premere "to press, hold fast, cover, crowd, compress" (from PIE root *per- (4) "to strike").
Meaning "action of weighing on someone's mind or spirits" is from late 14c. Sense of "whatever oppresses or causes hardship" is from late 14c. In Middle English also "rape."