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The Northern Isles were Christianised by Olav Tryggvasson in 995 when he stopped at South Walls on his way from Ireland to Norway .
 Olav Tryggvasson
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Olav Tryggvasson