The word "shit" is used 8-9 times in each episode. I'm different variants including bullshit, shithead, shitstorm, and shitbag.
Mild profanities like "damn", "bastard", "hell", and "ass" are used. Stronger language including "bitch", "goddamn", "dickhead", "asshole", "shit", and one use of "fucking" (used as a swear word, not sexually) are used too.
"Fucking" is used once as a swear word in the last episode of season 1.
The middle finger is used a couple of times throughout the show.
It should notice that it contains more strong bad language/swearing for TV-14 rated than anime series, and there's no use of "crap", "piss", "jackass", "dumbass", "badass", "smartass" and "balls".
Profanity gets stronger in each episode. Season 1 episode 8 has the most amount of profanity with 18 uses of shit, a use of fucking, multiple uses of son of a bitch and more intense profanity.
Several uses of goddamn it, damn, what the hell, dick, asshole, and son of a bitch.
Shit is said just a few times at the start of the series but increases more frequently in each episode. Season 1 episode 8 has the most uses of shit.
Insults/Name calling: Characters calls each other a "sniveling runt", "asshole", "bitch/son of a bitch", "scum", "shithead", etc.
In the beginning of episodes are mild with "hell", "damn", "bastard", "ass", and "bitch/son of a bitch", but it became moderate in later episodes with "dick/dickhead", "goddamn", and "asshole".
The language in One Piece is relatively mild compared to other shows with a similar rating. However, some profanity is used, though it is not prevalent.
"God," "hell," "damn," "ass," "bitch," "bastard," "dick," "asshole," "shit," one use of "fuck" and on-screen uses of "the middle finger" respectively, with the first time in One Piece franchise with final use of "fuck" in the Season 1 finales.
WAY more swearing than the anime.