Open source software, like free software, is a kind of software whose source code is available for inspection or modification. Some open source software is available for a fee, but much of it is available at no cost. To read more about free and open source software click here.
Open access is a kind of access or availability. This kind of access could apply to any digital content, such as software, music, movies, or news. But we only calls for open access to a certain kind of scientific and scholarly literature. To read more about open access click here.
(From the Budapest Open Access Initiative: Frequently Asked Questions)