You can oven roast a frozen turkey, but don't grill, deep-fry, microwave or smoke one.
Grilling and deep-frying use high temperatures that will quickly cook and char the outside but leave the inside of the bird only partially cooked, increasing the risk of foodborne illness. Microwaving also isn't a safe option because it cooks a frozen bird unevenly.
Smoking uses temperatures that are generally too low and take too long to fully cook a frozen turkey, also increasing the risk of food poisoning.
Oven bags aren't recommended for frozen turkeys either. At some point you need to open the bag to remove the giblets, and this allows contaminated juices to spill out. Opening the bag also releases scalding hot steam that can burn you.
To determine the approximate cooking time for a frozen turkey, follow this guideline from the U.S. Department of Agriculture (USDA): Take the recommended cooking time for a thawed turkey and multiply it by 1.5 for an unthawed bird. For example, if a thawed turkey needs to cook for 5 hours then a frozen one needs to cook for 7 1/2 hours.
Cooking times are usually listed on the package. You can also find recommended times for roasting whole turkeys on the USDA website.
Remember that roasting times are approximate, so the best way to know a turkey is fully cooked is to check the internal temperature. The entire turkey — including the stuffing — must reach an internal temperature of 165 F (74 C).
To check the temperature, insert a food thermometer in the innermost part of the turkey thigh and the thickest part of the turkey breast. After the bird has reached 165 F, take it out of the oven and let it stand for 20 minutes before carving.
A whole turkey usually has a package with the giblets and neck tucked inside. If the giblets are wrapped in paper, which is the case with most whole birds, there is no safety concern if they cook completely inside the bird.
If the giblets are wrapped in plastic, however, they need to be removed. It's difficult to remove a giblet package from a fully frozen turkey. So wait until the turkey has sufficiently defrosted during cooking and use tongs or forks to carefully remove the package. Then you can cook the giblets separately if you wish.
If the giblets are packaged in a plastic bag and the plastic melts, harmful chemicals may spread from the plastic into the turkey and the giblets. If you suspect that a plastic bag has melted inside the turkey, you must discard the entire turkey.
Only buy frozen pre-stuffed turkeys that have a USDA or state mark of inspection on the package, which indicates that the turkeys were processed under controlled conditions.
Don't thaw frozen pre-stuffed turkeys before cooking. Doing so takes too long and increases the risk of foodborne illness.