Cook bacon in a skillet until crisp, about 5 minutes; drain on paper towels. (Or microwave bacon according to package directions.)
Heat a grill pan over a medium flame. Lightly brush olive oil on both sides of bread slices and cook in grill pan (in batches if necessary) until toasted, about 2 minutes per side. Transfer toast to a board.
Stir together mayonnaise and pesto; spread on 1 side of each slice of toast. Top 2 slices with yellow tomato. Break bacon and arrange on top of tomato; follow with 2 leaves of arugula on each. Cover with 2 slices toast, pesto side down, and then spread toast tops with more pesto-mayonnaise. Top with red tomato slices, mozzarella and remaining arugula. Finish sandwiches with remaining toast, pesto side down. Cut in half and serve.