What is Martin Van Buren most famous for?

Martin Van Buren was famous for being the first American President who wasn't born a British subject. By the time that Van Buren was born, in 1782, the United States had been independent from Great Britain for six years. Van Buren also has the distinction of being the first President of non-British ancestry. He was actually born Maarten Van Buren to a family of Dutch Americans. Young Martin was raised to speak both Dutch and English; in fact, English was his second language, another first for a US President.
Talking of the English language, it's generally believed that Van Buren is indirectly responsible for the first written instance of the word "O.K." Van Buren was nicknamed "Old Kinderhook," after the town where he was born and where he would also die. Some of his supporters formed themselves into a group called the "O.K. Club," named after "Old Kinderhook." Sadly for Van Buren, it didn't help him get re-elected, but it has provided the English language with one of its most widely-used expressions.