I am running prefect UI on my machine and I have one task which should run on another machine (OS) in my office. Is it possible to use agent to run a specific task on a specific machine

Yes. you should be able to target different agents via run configurations https://docs.prefect.io/orchestration/flow_config/run_configs.html Assuming your other machine runs the appropriate prefect agent configured to look for your prefect server that you will register your flow with.