It will be easier to answer this question if a more specific scenario is given.
Let me try to suggest one:
A: 能不能再加一小时班? 明天绝对不会再让你加班了 (Could you work overtime again for an extra hour? I won't ask you tomorrow, I promise.)
B: 好吧... 既然你已经这么说了, 那我还能说什么呢? (O--k--ay--, If you say so... :-| )
If you simply want to say "O--k--ay--" in Chinese, with special tones, you can do it like this:
B: 唉~~~ (extend the final sound "Sigh" to express disappointment ) 好吧, 好吧, 好吧... (repeat saying "Okay" to show decent impatience in your tone .),
For other possible scenarios, please refer to the following links and examples:
- 10 ways to express your "dismissive feelings" in English (10 种英文表达说出你的无所谓)
- Some ways to express "whatever" (“无所谓”的几种表达)
- 对……无碍,不要紧 ~ be ok (fine) with
Whatever you think is fine with me. 随你怎么想,我无所谓.
永远用于否定形式, He couldn't give a damn whether he passes the exam or not. 他对考试及格与否满不在乎.
- 随你的便(按自己的意愿行事). ~ suit yourself / as you wish / if you want.
a) You don't want to join the club? Oh well, suit yourself. 你不愿意参加俱乐部是吗? 那好,随你的便吧
b) As for your departments plan, you can change as you wish. 至于你们部门的计划,你愿意怎么改就怎么改吧.
- 听你的 / 由你决定 / 取决于你. ~ it's up to you.
So if you really want class to make a difference in your life, it's up to you. 所以如果你真的想通过这门课改变生活,一切取决于你.
I don't care what she thinks. 我不管她怎么想.
- 管他呢 / 爱咋咋地 / 该怎么着就怎么着吧 ~ whatever
I totally have no idea how to file my tax return, whatever. 我完全不知道该怎么报税,唉,该怎么着就怎么着吧/爱咋咋地. (在中文口语里, "咋"可以跟"怎么"互换)
- 没什么好说的/没啥好说的. ~ Nothing to say.
I have nothing to say about the government's stimulation plan. 对政府的刺激计划, 我没什么/没啥好说的. (在中文口语里, "啥"可以跟"什么"互换)