Olivie Marie Olsdatter Ostrem b April 1878

Hi - I am looking for information on my grandmother who was baptised in Borgund parish in April 1878. Her mother's name was given as Ingeborg Martinusdatter and it appears that Olive was illegitimate but they do give a father's name, Ole Gundersen. I have no further information except it appears that under the name Ingeborg Rasmussen Ingeborg got married in about 1883 to Knud Larsen in Borgund parish. I was wondering why her name was given as Rasmussen on this and also if Olivie had siblings. She took the name of the farm Ostrem as her maiden name and Olivie and her husband Lars Hansen Rovold immigrated with two small children to US in about 1900. I can not find more information on my great grandparents ie Ole Gundersen's parents and Ingeborg Martinusdatter's parents. Ostrem appears to be a farm name where both Ingeborg and Olivie lived, but Ole Gundersen was from Dybvik parish. Whether he was the real father or not I do not know, he was 52 and Ingeborg was 34 when Olivie was baptized. More than one witness had the last name of Rasmusdatter. If anyone out there has a common ancestor I would love to hear from you.