Scatter onion in the bottom of a large slow cooker then top with chicken.
Season with oregano, salt, and pepper.

Pour over soup and broth then add thyme and bay leaf.
Cover and cook on high until the chicken is cooked through, about 3 hours.

Discard thyme and bay leaf, then shred chicken with two forks.
Stir in celery, carrots, peas, and garlic.
Cut biscuits into small pieces, then stir into chicken mixture.
Spoon liquid over any biscuits at the top.

Cook on high until vegetables are tender and biscuits are cooked through, about 1 hour more.