The biggest question is what type of program is being enabled. The OPM providers are helping with both degree and certificate programs. The majority focus on non-traditional students (i.e. what has become the majority of US higher ed), as working adults often working on degree completion or professional development of some sense.
The sweet spot of the market has been masters programs, but there are some OPM vendors also working with associates and bachelors programs. Certificates and direct ties to employment is a growing field, but even there there are differences – entry job skills, prof dev, career advancement.
The other big question is what is the type of business model. There are five big players, in terms of number of clients and revenue, that most people know about. These are full-service, mostly tuition revenue-sharing. There are a growing number of similar companies who are not (yet) as large as the big 5. There are also firms taking a different approach, more ala carte and fee-based instead of tuition revenue sharing.