Yes...natural essential oils are very effective at repelling fleas and ticks. There are many products on the market to consider, but since I make my own I can't recommend any.
Citronella and lavender are key ingredients for fleas, and palmarosa or rose geranium are key ingredients for ticks.
Keep in mind that the best flea prevention is health - fleas only infest unhealthy animals so if a dog is unhealthy and has a weak immune system, fleas may still be able to set up shop. Any dog however can get a flea or two and this is easy to manage with natural essential oils.
Ticks are opportunistic and will drop on anything it can - so the oils help to prevent them from attaching. I have a dramatic difference in the number of ticks I pull off my dogs on days I don't spray them with my concoction, than on the days I do.