The OTO test is really for a quick daily check of whether there is chlorine or not. Since it is a TC test it doesn't give you much more info than that. The FAS-DPD test should be done weekly as part of your testing schedule and the OTO test done in between as a quick check that there are no problems
Perhaps this will help:
Colorless = No chlorine, break out the big kit and figure out why
very pale yellow = not enough chlorine, break out the big kit and figure out why
Yellow to dark yellow = yow have chlorine and if your weekly tests are ok you are done
Orange to bronw = very high chlorine. Unless you are shocking the pool or running a high chlorine pool because of high CYA it's time to break out the big kit and see what's wrong
Notice that I do this strictly by color and not by numbers. A good way to learn the colors is to do an OTO test each time you do an FAS-DPD test and soon you will learn that when your FC is at your 'normal' range your OTO test is, say, a deep yellow. You now can tell if your chlorine is too low, too high, or in the ballpark on a daily basis from the OTO test. Just remember to do the big test weekly so you can make sure you don't have any CC and to get your exact numbers.
If you are trying to figure out how much chlorine you need to add daily or every other day use the big test until you know that "my pool needs (fill in correct amount) cups of bleach daily to maintain the level". Then when your chlorine shows low by the OTO test on a daily check add the chlorine. You will be amazed at how close you will be to the right amount. Remember, it's a pool and not a science experiement. While proper testing IS very important it is NOT necessary to micromanage your pool!
It's not really necessary to use the big kit daily but you should check pH and TC with the small tester at least every few days (daily is best).
IF you know that there are problems and you are balancing the water don't rely on the OTO test, use the FAS-DPD test.
OTO is a great tool once you learn how to use it!
