The oven method takes longer to soften the bread than the microwave method but works just as well. Steps:
Preheat your oven to 300F (148.8C).
Tear off a piece of aluminum foil large enough to cover and wrap your bread loaf/slice in.
Wrap your bread in the aluminum taking care to make sure it is completely covered.
Place your foil covered bread into your warm oven for 5-20 minutes. If you are just heating a slice or small roll, you'll want to warm it for less time (closer to 5 minutes) than if you are warming and remoistening a larger or thicker loaf of bread (closer to 20 minutes).
Remove your bread from the oven and allow it to cool within the foil. You'll want to let your bread remoisten inside the foil so that as it cools it doesn't release it's remaining moisture as steam.
Serve and eat as soon as it's cool enough to touch.
If your bread is really dry, I occasionally dab a couple drops of water along the length of the bread loaf with my fingers. This will help to reincorporate moisture into your loaf beyond just reheating and softening. If you decide to add water, make sure that you don't over saturate your bread loaf otherwise you'll end up with mushy bread. Start small, you can always add more.