Mercury poisoning in cats is rare, so, occasional tuna treats are probably safe for them. But, tuna-based foods should definitely not be your cat’s daily treats. A safer option for your cat would be goat milk or bone broth.
A small amount of tuna flavored oil will probably not be harmful for your cat, but, it is better to avoid it. Tuna in oil has high fat content, so, it is likely to upset your cat’s stomach and cause diarrhea.