In most tier lists by the Japanese, Order Sol is usually ranked one tier above Sol or at least equal tier at worst. Early versions of the tier list had Sol ranked very high, but that's to be expected when his combos are easy to do and produce good damage for very little effort. The game has evolved since then, and Sol's rank in the tier lists has generally dropped due to other characters' potential growing or being realized.
Personally I would have to say OS is more solid of a character than Sol is. While OS doesn't have the damage that Sol has, he's faster, has better normals, has a better mix up game, and has better match ups compared to Sol.
I've never heard or seen Sol killing anyone in one practical combo (practical being the key word). I can see potentially 100% dizzy combos on certain characters, but without a dizzy, a lot of meter, and/or guard meter up, Sol cannot do practical 100% combos. Currently the only character that can kill anyone or almost anyone in the cast with one combo would be Eddie.
Sol's worst match up would probably be either ABA or Eddie (probably 4-6 for Sol). Everyone else that gives him trouble would probably be a 4.5 against him at worst.