What's inside an OFX file
An OFX file is a structured text file (SGML in older versions, XML in newer ones) that describes account details and a list of transactions — each with a date, amount, type, and description. It's meant for software to read, not people, which is why it looks like tagged markup if you open it in a text editor.
Because the structure is standardized, any program that understands OFX can import the same file — that's the whole point of an open format.
What opens an OFX file
Personal-finance and accounting software: Quicken, GnuCash, Microsoft Money successors, and many bookkeeping tools import OFX directly. You typically use the app's File → Import (or bank-feed import) rather than double-clicking the file.
If your software wants a different format — QuickBooks prefers QBO, a spreadsheet wants CSV — convert the OFX first rather than trying to force the import.
OFX vs QFX vs QBO
QFX is Quicken's branded version of OFX — the same base format plus a Quicken-specific marker. QBO is QuickBooks' branded version, which also carries an INTU.BID bank identifier QuickBooks uses to recognize the account.
Practically: OFX is the generic open format, QFX targets Quicken, and QBO targets QuickBooks. Converting between them is usually a matter of adjusting those identifiers, not rebuilding the data.