1. HO
2. HOR (Biblical mountain)
3. HOUR
4. HUM
5. HUR (Moses' brother-in-law)
6. MHO (unit of conductance)
7. MO (moment —Random House Unabridged Dictionary ©2006)
8. MU (12th letter of the Greek alphabet)
9. MUR (Austrian river)
10. O
11. OH
12. OHM (unit of resistance)
13. OM (used in mantras)
14. OR
15. OU (Hawaiian bird)
16. OUR
17. RHO (17th letter of the Greek alphabet)
18. RO (an artificial language for international use that rejects all existing words and is based instead on an abstract analysis of ideas —WorldNet®)
19. ROM (male gypsy)
20. RUM
21. UH
22. UM
23. UR (Abraham's erstwhile home)