This is the wikipedia page on the OA.
Anselm's second argument
Anselm in his Proslogion 3 made another a priori argument for God, this time based on the idea of necessary existence. He claimed that, if God is that than which no greater can be conceived, it is better to be necessary than contingent; therefore, God must be necessary. To sum up:
1. God is the entity than which nothing greater can be thought.
2. It is greater to be necessary than not.
3. God must therefore be necessary.
4. Hence, God exists necessarily.
In Chapter 2 of “The Existence of Nature and God” Anslem′s Argument for the Existence of God is as follows:
1. God is something than which nothing greater can be thought.
2. It is greater to exist in reality and in the understanding than just in understanding.
3. Therefore, God exists in reality