In a 3.5-quart soup pot on medium-high flame, heat olive oil until shimmering (about 1-2 min).
Add sausage pieces and sauté until lightly browned; transfer cooked pieces to a plate and set aside.
To the heated, oiled pan, add onion, garlic, and celery; sauté until fragrant and slightly tender, stirring as needed (3-4 min).
Add orzo; sauté to lightly brown (2-3 min).
Add chicken broth, water, diced tomatoes (with their juice), Italian seasoning, oregano, fennel, salt, and pepper.
Bring to boil; continue boiling until pasta is cooked through (about 10 min); add sausage back to pot about halfway through boiling process.
Serve warm.
