ORS § 199.780
When it is desired to form a new county out of an unincorporated area or one or more contiguous cities or to change the boundaries of a city-county and an existing contiguous county, a petition praying for the formation of the new county or for the change in the boundaries of the city-county and the contiguous county shall be presented to the governing body of each city-county or county affected by the proposed formation or boundary change. The petition shall describe the territory proposed to be formed into the new county or transferred from the city-county to the contiguous county, together with the name of the proposed new county, if the petitioners seek formation of a new county. The petition shall be signed by a number of qualified electors registered in the territory to be formed into the new county or registered in the territory to be transferred to the contiguous county after the change in boundaries that is equal to 15 percent of the votes cast within such territory for all candidates for Governor at the election at which a Governor was elected next preceding the filing of the petition.
ORS 199.780