Use Washing Soda in hot water and let them soak for a few hours, even days if you don't happen to get to them right away.
I used a 19 gallon storage tote, holding 2 cases of bottles. The longer the soak, the less you have to do. Most labels should float off within an hour or 2. Then scrub the outside of the bottles with a stiff brush to get rid of dirt and glue remnants, if any, and brush the inside with a bottle brush, and rinse well. Those jet bottle washers that attach to your faucet (regular faucets, not the fancy kinds) are great! Let them dry, or sanitize and bottle away.
Washing Soda (Sodium Carbonate) is the leftover component after "Oxiclean" has lost its oxygen, at less than 1/4 of the price. It's also sold as "Laundry Booster" and used to make up 80-90% of powdered laundry detergents.
You can use Oxiclean in a pinch, a tablespoon or so per gallon, but the oxygen component goes to waste, it does nothing for soaking the labels off. Let soaking and time be your friends.