Is it considered offensive to address one's seniors by name in the US?
No, in the US, it's usually not rude to address people you know more than in passing (e.g. are colleagues, classmates, or in their class) by their first name. NB: The question isn't &quot;How should I address a professor?&quot; it's about general rules and behavior, especially (my interpretation) the same university.
In the US, professors, staff, and PhD students would address everyone on their own &quot;level&quot; by their first name. In every department I have encountered, anyone in any of those groups would address each other by their first name, but as others have pointed out, occasionally Master's students maintain a bit more formality.
Occasionally, some professors prefer their undergraduates (especially in large classes) to use their title, whether &quot;Professor,&quot; &quot;Doctor,&quot; or &quot;Mister/Miss/Mrs.&quot;
All undergraduates would use first names, and would almost always use first names with staff and graduate students they are familiar with.
However, it's typically considered polite to start using the title (for faculty, and maybe staff) and to only use their first name when invited, which most people in academia in the US do right away. Often, the convention is people sign their emails how they want you to address them. In my experience, it would be a raised eyebrow to use a first name where it was &quot;incorrect,&quot; but would really only elevate to &quot;rude&quot; once someone has consistent disregarded someone's expressed wishes.
In regards to your example, it would strike me as incredibly rude if  a professor took offense at another professor by using their first name.