Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
observātor, ōris, m. [observo], a watcher, observer (perh. not ante-Aug.): nemo observator, nemo castigator assistet, Plin. Pan. 40: observator et custos bonorum, Sen. Ep. 41, 2: Catholicae legis, i. e. one who obeys it, Cod. Th. 16, 5, 1.