What are the differences between O and Eb-1A visas?

Both O and Eb-1A are visas for alien of extraordinary ability. Although their requirements are similar and overlap each other, they are different visa categories. The major differences between these two categories are: 1) O is a non-immigrant visa while Eb-1A is an immigrant visa (green card); 2) O needs employer sponsorship while Eb-1A does not.