Can oxalic acid in spinach ruin my teeth?

Depends: Acidic foods can, over time, break away the enamel of your teeth. As long as you eat these foods in moderation your teeth will be fine. Spinach isn't a big offender.

Unlikely: Any acid can demineralize teeth which is not a good thing. That being said, oxalic acid is a weak acid and the benefit derived from eating spinach far outweighs any harm to your teeth. Citric acid found in fruit juices is much more of a worry.

No. : Fresh/raw spinach leaves are good for overall health. So far we do not know of any problems to teeth when spinach is eaten in its raw/fresh state. Taken moderately and as long as you are not allergic to them, fresh spinach leaves are beneficial to us. They contain chlorophylls, fibers, and more.