OC, otherwise known as "overclock", is an important feauture in the game that allows troops to cost lower than usual, depending on the amount of OC it has. 30+ OC is the "max" overclock, and can decrease the cost of a unit significantly.
Max OC per Level-
1*- 5 OC
2* - 10 OC
3* - 15 OC
4* - 20 OC
5* 30 OC
(At 30 OC, a unit that normally costs 35 Unit Cost will only cost 24 Unit Cost!)
Methods of Max OC-
The most commonly used method is to farm 1-star units, usually from the Non-Event Stages. Most units can be found in these stages and can be farmed fairly easily.
1) To Max OC a 1-star unit, simply obtain 6 of the same kinds of unit, and "Enhance" one of them with the other 5. This will raise your OC level to 5, thus maxing the OC for a 1-star.
2) After maxing the OC of a 1-star troop, max out the level so you can level it up to a "2-star" unit. (Seperate troops must be obtained in order to Evolve the unit.)
3) By repeating steps 1 & 2, you will end up with two, 2-star units. You can now enhance those togethor to max the 2-star units OC at "10". (Note- Only units that have the same star-level can raise the OC level. OC levels add up.
If you repeat these steps, you will eventually end up with a Max OC 5-star unit!
(Remember, OC adds up, so an OC 10 unit enhanced with another OC 10 unit can make an OC 20 unit.)