Can oxalic acid in spinach ruin my teeth?

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.
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.