Does Luffy Have Observation Haki. If playback doesnt begin shortly try. This first happens during the beginning of the Sabaody Archipelago arc.
But even humor has a thread of truth in it. Luffy had to close his eyes to force himself to rely on his observation instead of his vision. Yet this one time he saw his arms getting cut off.
From here Luffy also makes use of Observation Haki before learning to hone it.
Not quite certain if it was the very first but he used some sort of Conquerers Haki against Motobaro Duvals bull in Anime episode 388. Luffy an zoro were facing him head on and both of them didnt use their observation haki luffy can literally see the future to see what was gonna happen and stop it b4 it happened kinda like. Zoro and Sanji have Observation and Armament. Like when Luffy and Zoro sensed more than one person in Blackbe.