Is there an over-the-counter treatment for scorpion sting?

Depends. If the sting is caused by a non-toxic scorpion, then pain meds and wound cleaning materials would be otc, otherwise no.
Let advise you. Scorpion stings are most serious in young children, older adults and pets. In the United States, healthy adults usually don't need treatment for scorpion stings, but if your child is stung, always get immediate medical care.