He uses it to find the strongest opponent on the battlefield (sensing Kaido’s monstrous presence). He uses it to protect his crew (feeling when Tama is in danger). He uses it to time his most devastating attacks.

If you type "Can Luffy use Observation Haki" into a search bar, the short answer is a resounding yes . He’s used it dozens of times.

What do you think? Does Luffy’s lack of constant Future Sight hold him back, or does it make him a more interesting fighter?

His formal awakening came during the brutal two-year training on Rusukaina with Silvers Rayleigh. By the time he returned to the Sabaody Archipelago, he was a certified user. We saw it explicitly when he dodged the Pacifista’s lasers with a calm smile, and again when he sensed the presence of 100,000 Fishman pirates on Gyojin Island.

So why isn't he untouchable? Here is the deep cut: Luffy has the potential for god-tier Observation Haki, but his personality actively nerfs it.