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Usually, you would add them to the side that needs oxygens. For example, reactions that involve MnO4 or Cr2O7, typically require the addition of water to the other side so that the number of oxygens are balanced. Use OH- when dealing with basic solutions. You would still add it to the side that needs oxygens to balance the equation appropriately.
I add H2O and OH- on the right side for reduction and then balance both sides by adding H2O to the other side. For oxidation, add H2O and OH- on the left and balance the number of H by adding H2O on the right. That's assuming you need to add any oxygens or hydrogens.
As long as the charges and number of atoms are balanced, meaning that the number of atoms on one side equals the number of atoms on the other, the equations are balanced. OH- is only added in cases where the reaction takes place in a basic solution.
For balancing basic reactions, add H2O's to balance oxygens, after that add H+'s to balance the hydrogens and finally, since this is basic, for every H+ you have added add that same amount of OH- onto both sides and note that OH-+H+=H2O, so there will be some cancelling.
You can also treat basic solutions like acidic ones by adding H+ and H2O as you would, and then add the same amount of OH- as the H+ (and add that amount of OH- to the other side as well). On the side where there's H+ and OH-, you can combine them to get H2O and then make sure the H2O is only on one side of the reaction.
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