An oppressor is any authority (a group or a person) that uses its power unjustly to keep people under control. Many rebellious teenagers view their parents as oppressors, but the word is usually used to refer to dictators.
While you might complain about your oppressor, the teacher who makes you stay late to catch up on assignments you missed, a true oppressor denies basic human rights to people who live under their control. Before Partition in 1947, the citizens of India mostly viewed their British rulers as oppressors — in some cases, Indian farmers were forced by the British to grow non-food crops, causing thousands to starve. The Latin root is opprimere, "to crush or subdue."