Many "Hindi Dubbed" versions found on free streaming sites or YouTube are fan-dubs with poor audio quality and mistranslations. Since Oldboy relies heavily on dialogue for its mystery, a bad dub will ruin the experience. We strongly recommend using official platforms.
You might want a structured overview of the film (plot, cast, and availability) specifically for Hindi-speaking audiences. Oldboy Korean Movie Hindi Dubbed
However, here are some legitimate avenues to explore: Many "Hindi Dubbed" versions found on free streaming