Heat oil in a small saucepan over medium heat. Cook garlic for 30 seconds, add tomatoes, oregano, salt and pepper and simmer for 10 to 12 minutes until thick, stirring frequently.
Heat the broiler. Put bagel halves cut side up on a sheet pan and toast for 1 to 2 minutes until the cut faces are pale gold.
Spread the reduced sauce evenly over the four bottom halves, leaving a small border. Add pepperoni, then cover with half the mozzarella.
Divide remaining mozzarella over the four top halves. Broil all halves cut side up for 2 to 3 minutes, watching constantly, until cheese melts and begins to brown in small spots.
Use a spatula to turn each cheese-covered top onto a filled bottom. Rest for 2 minutes, then cut each closed sandwich in half and serve while the cheese is soft.