It is a well known law yet often over looked by home owners and REALTORS® alike. In the State of Oregon, when a home transfers it is the seller’s responsibility to be certain all smoke alarms met the current fire codes. The current fire code in Oregon basically states that all smoke alarms must have a hush feature. 
The good news is…. only alarms with a hush feature are available in Oregon Stores, the confusing news is – hardwired alarms must be replaced with hardwired alarms. So, if you have a hardwired alarm you can not replace it with a battery operated one. The link above is a great resource to answer your questions.
If you have an offer on your home – the home inspector will check your alarms, and also verify if they are up to current code or not. Although some feel that this places the REALTOR® in the position of enforcing the law, I personally feel good knowing my clients are safer because of it.
While reading what other Real Estate Agents and Brokers had to say about this I did run across an article by Lynda Eisenmann on her livinginsocal blog that states the following. I am thrilled to see other REALTORS® take the time to educate and help their clients be safe.
1. Smoke Alarms. Install alarms on every level of your home and in and in every sleeping area. Change batteries once every year; test them monthly; replace the units every 10 years. Smoke alarms can cut your family’s chances of dying in a fire by nearly half. Plan escape routes and conduct fire drills with the entire family. Find two ways out of each room.