Hi Brian,
Welcome to the forum! 
It is safe to give RO water to pets on occasions. The general rule is, if the water is safe for you, it should be safe for pets.
However, RO water lack of minerals. So it is not so healthy to drink it for long-term. It can "wash" away the minerals and vitamins consumed by the pets.
It is so nice of you to provide food and water to strayed cats. Did it keep coming back for more?
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