Where did you see they have high oxalates? I've never heard that. I know older leaves are higher in oxalates (per sandrachameleon), but I don't believe they're high enough to be a concern like other veggies can be. Dandelion leaves are generally regarded as excellent gutload material because the calcium ratio is high.
bkullman - I don't think you can ever really give them too much calcium so it wouldn't hurt to give something high in calcium, especially if you're going to substitute it for something with lower calcium. They're good at excreting excess calcium and always need high amounts of calcium for proper growth and metabolism. I give dandelion greens a few times a week!
Dandelion greens are generally considered to be excellent gutload for feeders and food sources for herbivorous herps. As for their oxalate content, they have <25mg/100g compared to the ~750mg/100g in spinach.