What's really weird is that he has an accent he's never had before.
Adamska was born in Normandy, France in 1944, before being kidnapped and raised in America until the age of sixteen. In 1960, Adam defected to Mother Russia and, I can only assume, spoke Russian indistinguishably from a native speaker as no one seems to question his loyalty*. In 1964 during the Virtuous and Snake Eater missions Ocelot is encountered by the player for the first time
chronologically, but now he speaks in a neutral TV American English accent. This could be because all the dialogue is "retroactively" being translated by our "film crew," or our Limited Third-Person Narrator. This, however, is confused further as Naked Snake spoke with his American English VO, as did the other reoccurring characters, so why would Ocelot be the only exception? That probably because he wasn't; all the random guards also spoke in generic branded American English.
As of 1964? I got no clue what Ocelot sounds like unless it's just insinuating "this is what this character would sounds like if they were speaking English." It seems like from there he just based it on whatever the last Spaghetti Western he had in his Metal Gear Blu-RAY player was. Honestly, Old Ocelot just sounds like a different brand of country western and villainy stirred together.
*I think this is right... It seems like everyone assumes he's Russian from my memory?
**Also give Troy a break! Kojima was chunkin' out songs he contracted left and right. I get the feeling if that performance wasn't what he asked for he'd milk what he wanted out of there.