Heat oil in a large saucepan over medium heat. Add bacon and cook until crispy, 4 to 6 minutes. Transfer half of the cooked bacon to a paper towel-lined plate with a slotted spoon. Add onion, celery and thyme to the pan; cook, stirring, until beginning to soften, about 2 minutes. Add potato, clam juice and bay leaf. Bring to a simmer, cover and cook until the vegetables are just tender, 8 to 10 minutes.
Whisk milk, cream, flour and salt in a medium bowl. Add to the pan and return to a simmer, stirring, over medium-high heat. Cook, stirring, until thickened, about 2 minutes. Add clams and cook, stirring occasionally, until the clams are just cooked through, about 3 minutes more.
To serve, discard bay leaf. Ladle into bowls and top each serving with some of the reserved bacon and scallions.