The Open Circle Fleet was a fairly simple piece of SW lore, but because of inconsistencies, it doesn't really make sense anymore. Regardless, I'd like to try to explain it.
The Open Circle "Fleet" isn't really a fleet at all. It's technically an Armada. An Armada comprises 10 to 50 fleets and is under the command of a High General. The Republic Navy consists of ten Armadas. Thus the Open Circle Fleet would account for roughly ten percent of the Republic's naval strength. The OCF was under the command of High General Obi-Wan Kenobi, and formed the spearhead of the Outer Rim Sieges.
The real continuity issues arose when The Clone Wars rolled in. In the show it seems as if all Republic vessels belong to the Open Circle Fleet. As I've explained this can't be the case. 10% means a lot of open circles, but not to the amount portrayed in TCW. Indeed, following the ORBAT it would be impossible for a High General other than Kenobi to command an Open Circle vessel; In TCW many High Generals prance around with ships from Kenobi's Armada. This simply isn't accurate and should be written off as a continuity error.
The Clone Wars is fairly unreliable as a source and I prefer to focus on older media.
Alas, the Open Circle Fleet or Armada is an interesting topic. It pays to read the Campaign Guides.
