Someone is full of it. I have worked at dealers for almost 25 yrs and we don't sell O2 sensors for any particular reason unless they're bad. Don't fall for that line as whoever that's telling you that, an inexperienced or dishonest person is simply wrong.
I can see one going but you'ld get a check engine light and fault codes will be stored. Even if there's a code for both O2 sensors at once, that's then usually another problem such as a vacuum hose leaking. Besides, a bad O2 sensor will never leave you stranded if one does go bad in the future.

Now, I'm sorry that you've run into someone who's not honest there but in my years of dealership experience, most are honest and wouldn't make up a story like that. Are you dealing with a new service writer perhaps who doesn't really know Volvos very well?