There is a death record for an Octave Nadeau at ancestry.com
He died Jan. 3, 1901 Westbrook , Maine age 42, born Levis, Quebec his parents shown as F. Havier Nadeau and Marie Tergeon, both born Quebec
BUT this fellow is shown as being single - not married. So while parent's names match what you though they might be, it does not seem to be the right fellow as your Octave was married
You did not say where your Octave died - was it in Maine?
Also you did not give the names of children of Octave and Arthemise - which might make it easier to find them.
Do you know approx. birth years for Octave and Arthemise?
Did I do some looking for you before on the Cote family as her mother's name seems familiarMarie Desneiges Pare.
** I just found Arthemise in 1871 census for Quebec - where she is age 10 with parents Paul 34, and Desneige 32, and siblings Joseph 8, Achille 7, Elmire 5 and Arthur 1 month.
Also found family in 1891 Ontario so looks like she and Octave went back there. Arthemise death is in the ON Catholic records 1902,but did not find Octave there - do you know where he died? His death record may show parent's names. Odd that their marriage doesn't show up I see in the ON Catholicrecords they also had a daughter Marie born and died 1884 so marriage should be within the 2 years before that I would think and maybe in ON but could not find it there.
There is a tree which shows Octaveborn March 5, 1857, dying 1900 in Chapleau, Ontario - but shows parents as Francois Xavier Nadeau and marie Turgeon - which are parents as shown on the death I gave you above for the Octave died Maine who was single
There is another tree which shows his father as Remie Nadeau and Octave as Emile Octave Nadeau both April 26, 1861
neither of these shows a marriage date or place and neither shows the death record There is a baptism for an Emile Octave Nadeau baptized April 26, 1861 but father is Pierre Nadeau and mother is Genevieve Martin (I think that is her maiden name, hard to read)
I looked for any marriages for an Emile, or an Octave Nadeau but still nothing - I tried Maine, Ontario and Quebec.
I'm leaning toward ON as their first child was born there but still no marriage shows up in either the ON marriages or the ON Catholic marriages.