What's the difference between "adviser" and "advisor" -- are both interchangeable?

I work for a financial services provider and we deal with "Financial Advisors" all the time. Increasingly, I'm seeing people send emails and so forth with the term "Financial Adviser" and the terms adivsor and adviser seem to be increasingly interchangeable.

Which then raises the question: what is the difference between adviser and advisor?

Both are right. This is how the Oxford dictionary entry explains the usage:

The spellings adviser and advisor are both correct. Adviser is more common, but advisor is also widely used, especially in North America. Adviser may be seen as less formal, while advisor often suggests an official position

I just happened to come across this issue writing a final exam for a course in international financial derivatives regulation. (For those of you who don't know what that is, it's what you do in purgatory.) Both spellings are correct. In general,"adviser" is the preferred spelling, especially in the UK. "Advisor" carries a connotation of someone whose professional capacity is to give advice. On that basis, and since my course had a strong focus on American regulation, I changed "adviser" to "advisor" regarding "very-high-net-worth individuals ... who tend to employ the service of sophisticated advisors to assess and manage their risk." If you want to know my sources, just google "adviser or advisor" like I did and you'll get them all.

Can't resist taking a swipe at the obiter on "amount" vs. "number" in the media. My two fields are English Literature and Law. Like Law, English grammar is based on rules which must be faithfully applied in order to preserve a unified fabric, but like Law, English is (forsooth) a living language. It lives and is preserved our media of communication. When the editors of the OED want to know what the "correct" usage of a certain term was in the 19th century, they turn to ... the media! I'm the first to be a stickler for good usage, but don't overdo it.

I got my hands dirty by digging through data about how our tech society handles one word with two spellings:

Advisor or Adviser: A data-journey for one word that goes both ways

tl;dr the spellings are interchangeable. Some publishers have style guides that require one spelling or the other. By counting usage, some leading words show more prevalence of O or E, e.g. jewelry adviser is much more used than jewelry advisor, and stock advisor is much more commonly used instead of stock adviser.

Here is a chart showing ratio of O:E spellings for many leading words:

In it you'll find polarized opinions, such as:

And also people who couldn't care less:

Advisor should be the preferred spelling, consistent with other official titles:

actor convenor coordinator editor regulator

I am a professional general and scientific editor with over 40-years experience.

The tragedy is that the media often perpetuates mis-spellings and mis-use, witness the current use of 'amount' for every quantitative description, e.g. amount [sic] of people, rather than number of people.

In Australian Financial Services, the legislation uses the spelling Adviser so that is the source of truth in Industry. In regular Australian vernacular both spellings are used.