上岗 (shàng gǎng) means 'to start working at a job' or 'to assume one's post' — it focuses on the moment of beginning a position, especially after training or qualification. Unlike 就职 (jiù zhí, formal 'take office') or 任职 (rèn zhí, 'hold a post' over time), 上岗 emphasizes the start of duty in a specific role, often in blue-collar or service jobs. 在职 (zài zhí) means 'currently employed' (a state), not the act of starting.