OTP is slang for One True Pairing, meaning your favorite combination of characters in a fandom.

Sci-ence.org does cool art about science and under the OTP heading they used a neat story about a mammal forerunner and an amphibian that lived (and died) together in an African burrow 250 million years ago. 

Well, I love everything about this so I am linking to it here. Make sure to click all the back arrows (or forward arrows if you are reading this after the next time Maki makes a post) and enjoy a lot of good stuff. You can even buy the prints.