Few people who say 'ok' reflexively to a question realize that the term comes from Van Buren.
He used to sign his letters 'O.K.' which stood for 'Old Kinderhook' (his hometown in NY)- one of his many nicknames. This one was use mostly by his operatives to give a homey feel to his policies (think Old Hickory for Jackson or the fireside chats of FDR)
For whatever reason, the phrase stuck around for much longer than people's memories of the president.
Some other interesting nicknames of Van Buren (he had many)
"The Careful Dutchman" (his native language)
"The Little Magician" (he was a kingmaker and very short)
"Martin Van Ruin" (Whig critics name for him due to the Panic of 1837)
"The Red Fox of Kinderhook" (the best kind of nickname, used by friends and enemies)