PH readings of RO Water, or any purified water for that matter will usually be around 6 - 6.5.
This is because the water is so pure that it actually absorbs carbon dioxide from the air, however, because the water is so pure it only takes a very small amount of buffer in order to make a large difference in value, and if you are adding it to existing tank water you probably wont need to do anything.
Distilled and RO Water is typically very low in free oxygen, so it is a good idea to AERATE your RO Water for 24 Hrs before using it