Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain on paper towels, reserving drippings in the skillet. Chop bacon into smaller pieces.
Combine hash brown potatoes, O'Brien hash brown potatoes, chicken bouillon, celery seed, and black pepper in a large pot over medium heat; bring to a boil. Reduce heat and simmer, covered, until potatoes are tender, 20 to 30 minutes.
Puree potatoes with an immersion blender until smooth.
Pour reserved bacon drippings, cream of chicken soup, and milk into the pot. Simmer until heated through, about 10 minutes. Sprinkle chopped bacon on top.