Luckily no, old French is not much different from modern French. The vocabulary tends to sound a bit medieval and some sentence constructs which are not commonly used nowadays were the rule back then, but I'd say if you can read modern french you can read old French without too many problems. The biggest problem is, indeed, the vocabulary of certain words which have now become completely obsolete.
The oldest French text known to date was, indeed, a weird mix of Latin and Gaul, with already transformed litteral structures. This text (very short) can be viewed here, accompanied with a modern french as well as english translation
http://www.historique.net/philologie/serments/
this, indeed, is some wicked text
but the "classical" old french is something that resembles more
http://abu.cnam.fr/cgi-bin/go?troylion1
A normal native frenchman could perhaps not understand it fully, but get a grasp on 80% of it without too much trouble.