This would appear to be your Olga:
1865 census, 0903-Arendal.
address: Tyholmen; town of Arendal.
First of two households:
---Marie Aanons., Housewife, house owner, widow, age 59, born in Arendal
---Johanne B. Aanens., her daughter, unmarried, age 29, born in Arendal
---Peterine Peders., servant girl, unmarried, age 30, b. Arendal
---Johanne M. Hendriks., Foster daughter, unmarried, age 13, b. Arendal
---Olga B. I. Peders., Foster daughter, age 10, b. Arendal
---Michael Lars., unmarried carpenter's apprentice, age 24, born in Danmark
---Martin Knuds., unmarried carpenter's apprentice, age 27, born in Mandal
---Collert Mikkels., unmarried seaman, age 28, born in Arendal
Second of two households:
---SÃ¸ren Terjes., seaman (mate), age 36, born in Arendal
---Christine Marie, nee Aanens., his wife, age 34, born in Arendal
---Theodore G. Marie SÃ¸rensd.*, their daughter, age 10
---Theodor Martini SÃ¸rens.*, their son, age 1, born in Arendal
[I believe that the * means that no last name was given originally; the name has been added.]
And her brother is living and studying with a painter in Grimstad, not that far away:
1865 census for 0904-Grimstad, Aust-Agder.
address: Vestregaden. Town of Grimstad.
---Lauritz F. Hanss., master painter, age 42, born in Christianssand
---Hanne Severine, nee SÃ¸rens., his wife, age 31, born in Grimstad
---Henrik Oscar Hans., son, age 13, b. Christianssand
---Bernhard Petter Theodor Hans., son, age 11, b. Christianssand
---SÃ¸ren Petter Hans., son, age 7, Grimstad
---Ingvald Paaskeberg Hans., son, age 4, b. Grimstad
---Hans Hans., son, age 2, b. Grimstad
---Johan Rudolf Petters., his [Lauritz'] pupil/apprentice, unmarried, age 18, born in Tronhjem
---Mathilde Gurine Jacobsd., servant girl, unmarried, age 20, born in Christianssand
---Hans Jacob Hanss., his [Lauritz'] brother, worker, carpenter's apprentice, unmarried, age 37, born in Christianssand
Here is a rather mystical Olga in 1900. She may not be yours.
1900 census for 0915-Dybvaag, Aust-Agder.
Municipality: Dybvaag (krets no. 3, gardsnr. 17, bruksnr. 1)
Farm name: Ã˜stre AskerÃ¸en
All are Norw. citizens and members of the state church.
---Knud Niels., ship owner and farmer, widower, born 16.10.1820 in this census district
---Olga Peders., housekeeper, unmarried, born 1854 in Holt Ned
---Andrea Jens., unmarried servant girl, born 1878 in Gyland LM
---Anna Peders., seamstress, unmarried, visiting here temporarily from LyngÃ¸r, born 1851 in Holt
[One wonders if Knud is related to your Niels Olsen? One wonders if the visiting Anna is Olga's sister?]
[I believe that Holt means Holt near Tvedestrand, where the Holt Church was built in about 1100.]
This looks like the same Anna in 1865, but it doesn't tell us anything about her family:
1865 census for 0914-Holt.
School district: NÃ¦s JernvÃ¦rk. Holt sogn (sub-parish), Holt prestegjeld (main parish). Farm name: Lien 1.
2 cattle, 1 pig.
All except the wife are born in Holt.
---Thor Gustavs., Digelarbeider med Jord, age 30
---Anne Axelsd., his wife, age 29, born in Fjekkefjord
---Axel Thors., son, age 7
---Gustav Thors., son, age 6
---Marie Thorsd., daughter, age 2
---Anne Pedersd., servant girl, unmarried, age 15
Hope this helps.