Scrub the littleneck clams thoroughly under cold running water to remove any sand or grit. Discard any clams that are already open and do not close when tapped firmly on the counter. Place the cleaned clams in a bowl of cold salted water for 20 minutes to help purge any remaining sand, then drain and pat dry.
Toss the clams lightly with the olive oil and Old Bay seasoning in a bowl.
Preheat the air fryer to 400 degrees F. Arrange the clams in a single layer in the basket. Cook for 6 to 8 minutes until the shells pop open. Check at the 6-minute mark and remove any clams that have opened. Continue cooking the remaining clams for another minute or two.
Discard any clams that still have not opened after 10 minutes.
While the clams cook, prepare the garlic herb butter. In a small saucepan over low heat, melt the butter and add the garlic. Cook gently for about 1 minute until the garlic is fragrant but not browned. Remove from heat and stir in the parsley, lemon zest, lemon juice, red pepper flakes, and salt.
Arrange the opened clams on a serving platter. Spoon the warm garlic herb butter generously over each clam.
Serve immediately with plenty of crusty bread on the side for soaking up the buttery juices.