According to this, "okay" is mediocre. http://www.cbsnews.com/news/time-for-a-little-word-history-ok/

Specifically, Mediocrity that Succeeds. Which, while less than perfection, is far from mediocre.

"It allows for mediocrity, but mediocrity that succeeds," Metcalf said. "Because Americans figure out how to get things done rather than waiting for perfection."

Thanks to this article, I now want an 1864 edition of the Slang Dictionary of Vulgar Words.