An OP amplifier including an input stage and an output stage for improving a slew rate is provided. The input stage receives one of input voltages, and generates an internal voltage according to the received input voltage. The output stage receives and gains the internal voltage, and outputs an output voltage. The output stage includes a first transistor, a plurality of first capacitors and a first switching unit. The first transistor includes a first source/drain terminal coupled to a first voltage, a gate terminal controlled by the internal voltage. The output stage outputs the output voltage according to a voltage at a second source/drain terminal of the first transistor. First terminals of the first capacitors are coupled to the second source/drain terminal of the first transistor. The first switching unit selectively transmits the internal voltage to the second terminal of a corresponding one of the first capacitors.