An optical multiplexer and a transmitter optical subassembly are provided that relate to the field of optics communications. A light-combining part is formed by using at least one light combiner that can combine two beams of light into one beam of light, and at least two levels of light-combining parts are used to constitute an optical multiplexer, so that 2N beams of light are combined into one beam by using N level of light-combining parts, thereby reducing a package size and packaging loss, and reducing the complexity of a manufacturing process.