Do you have to have carbon monoxide detectors when selling a home in Oregon?  Yes.  As of April 1, 2011 House Bill 3450 requires that carbon monoxide detectors be provided by the seller of Oregon real estate:

Home sellers of one-and two family dwellings, manufactured dwellings, or multifamily housing units containing a carbon monoxide source must have one or more properly functioning carbon monoxide alarms before conveying fee title or transferring possession of a dwelling… The law requires a carbon monoxide alarm on each level of your home with sleeping areas and within 15 feet of each sleeping area. Oregon Fire Marshal Carbon Monoxide Q & A.

Landlords are also required to provide them for rental dwelling units.  The five page Q&A explains what carbon monoxide is and what sort of detectors are acceptable.