O
- oaf
- alfr (="elf")
- odd
- oddi (="third number", "the casting vote")
- Odin
- Óðinn
- ombudsman
- from Old Norse umboðsmaðr through Swedish ombudsman, meaning "commissary", "representative", "steward"
- outlaw
- utlagi
Read more about this topic: List Of English Words Of Old Norse Origin