OTO will accuately tell TC. The problem with testing FC is that the color keeps darkening for about 2-3 minutes and when are you supposed to read the FC? I have seen kits that say read it in the first 3-5 seconds (a no name cheapie comparator I picked up about 8 years ago at my spa dealer), ones that say read it at 15 seconds (Walmart Aquachem), and ones that say read it within 30 seconds (a 4 way kit that I saw in a pool store, private label)...which one is right? They are all using the same chemcial for testing! I believe you can get an idea if you have CC with OTO IF the level of combined chlorine is high, but then again, I can smell that with my nose! If you want it to about 1 ppm then you need a DPD test, if you want it more accurate you need either a DPD test with a colorimeter or an FAS-DPD test. I do know that most states do NOT allow OTO testing for commercial pools and require either DPD or FAS-DPD for precisely this reason....you cannot accurately test for FC with OTO! The other problem with OTO testing is the differenced in the darkness of the yellow color are very small and hard to distinguish when they are less than 1 ppm apart! You can see this by looking at OTO comparators. The shades of yellow are very close. Even with TC I feel it is an approximation at best. (But OTO is best for a 'quick and dirty' check on your chlorine levels.