My dog ate topical flea medicine. Is it toxic? Should I see a vet?

It would be helpful to know what specific type of flea medication is was, since different brands have different active ingredients. But in general, if he only ate a single dose, I would not expect this to cause any serious problems - these medications typically have a wide margin of safety, even if they are ingested rather than applied topically. You may see some mild stomach upset, but I would not expect any serious toxicity issues. To be safe, you should monitor for any neurologic issues such as muscle tremors or seizures, and take Oscar to the vet right away if he is acting abnormal.