Business schools frequently offer PhDs in a subset of the following areas…
- economics
- marketing
- finance
- statistics / econometrics
- organizational behavior
Not to be harsh but…
1. I don’t think most reputable places would let you do it all online, nor would that be a good idea.
2. I also think your understanding of the degree is wrong if you are thinking about it as “low hanging fruit.” These degrees generally take 5-6 years to complete full time and are designed to train researchers to produce research articles in the areas above. It is a full time job (or more) anywhere.
3. Neither your MBA or your business experience are likely to be that relevant.
There are some purely online schools which have sort of “pay to play” phd programs. You pay tuition and they pretend to teach you something. Probably the first result that pops up when you Google “online business phd program” will be a good example. I don’t think those are likely to be a good investment of your dollars, nor are they taken seriously in the research community.