What's the difference between “adviser” and “advisor” — are both interchangeable?

Question

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?

Answer 1

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

Answer 2

The nearest reference book to my computer, the "Oxford Writers' Dictionary", says

advis/er not -or; ory