My dog has been sneezing for days. What can I safely give him OTC?

Unfortunately no, there isn't anything I can recommend you give Simba that OTC. By law it is illegal for the experts at Petco Pet Education Center, formerly Petcoach to suggest any drug, including human OTC drugs, without seeing the animal in person first. That's because doing so requires a veterinary client patient relationship which cannot be achieved via an internet forum. In addition, the experts cannot say for certain if a specific drug is safe for your dog. You can try using a humidifier or bringing him into the bathroom with the door closed while you take a hot shower if he's congested- the steam and humidity help to break up congestion and temporarily relieve symptoms associated with respiratory infections. Your other option is to take Simba into the vet, who can prescribe him medications to help with his sneezing and symptoms.