Best flea medicine (oral) for dogs?

There is no such thing w/o vet script.  Sorry you have a problem APPLYING the topical, but perhaps some advice.  BTW, let's keep in mind - you OWN the dog, not the other way 'round.  

I have dogs weighing 85 & 90 lbs.  I simply have somebody hold the dog by the collar, or  I "tie" the dog to a doorknob, while I "dot" the liquid application from shoulders to tail (about 4 spots).  You would be surprised how LITTLE a dog can 'dance around" if you sort of pin the dog to a wall or fridge (with your legs) while tied up.  I use Advantage or Advantix.  In my area, fleas have become IMMUNE to Frontline (since it was the very first product put on the market as a spot-on.)

If you slip one HAND in the skirt area & just touch there (between hind leg & stomach) that is the 'freeze spot" for 5-8 seconds where dogs often nose or touch other dogs, BEFORE going in for a "sniff".  It does not last long, but will HELP cause your dog to be STILL (briefly). Have product open and READY to ROLL!

Once done, make a big to-do over the RELEASE and if it goes (even remotely well) give a treat.  Do not wash dog for 2-3 days as per the INSTRUCTIONS with your product.

There are no over the counter flea medications that are oral that can prevent fleas long term. Your dog would need to see a vet for that. If you are worried about the cost of a vet, look for vaccine clinics in your area. They generally post their prices online for physical exams. They can also get you some oral flea medications for your dog. Can you put a flea collar on the dog? Many people have success with the Seresto flea collar. 

SENTINEL® SPECTRUM® Chews or Triflexis.

There are no over the counter oral flea meds.   They are RX only.

If you see the vet yearly (as you SHOULD), then you should easily buy this from your vet without needing an appointment or exam

I bought over the counter ONCE and my dog had a seizure.  Can't afford a Vet?  Some other problem?

I wouldn't EVER give a dog an over the counter brand.

The only OTC flea med I have seen is called Capstar but after reading the information it is for treating a flea infestation, and it kills fleas for only 24 hours so it isn't a preventative like the prescription meds that you give once per month. It also doesn't treat ticks

I will only use front line. There are problems with the oral meds that can cause permanent damage to your pet's health.