Danish F-16 military aircraft scrambled to escort the plane to Norway, The Danish Defence Command said on its Twitter account.
The situation is under control and police have arrested a 51-year-old British man on suspicion of being behind the threat, the police said in a statement.
The aircraft will now be examined by the emergency squad and bomb group, the police said, adding all passengers were safely evacuated.
“Crew on a Ryanair flight from London Stansted to Oslo today discovered a note that claimed there was a potential security threat on board,” a Ryanair spokeswoman said in an email to Reuters.
“The captain followed procedures and continued to Oslo airport where the plane landed safely. Customers remained on board while authorities carried out a security search,” she added.
