OBML files contains website content stored in compact binary format used by Opera Mini web browser (versions 3 to 5) called Opera Binary Markup Language. OBML format is unique to this web browser and can be opened using either specific vesion of the Opera Mini or software that emulates the browser.

It is possible to open OBML file on a computer instead on a mobile device by using Opera Mini emulating software – OperaMiniLargeDevice or MicroEmulator. With the release of Opera Mini version 5 OBML files have been replaced by OBML15 file format which is not backward-compatible, although based on the same file standard. OBML15 offers significant gains in terms of page rendering speed (up to 90% increase when compared to OBML format) and reduced bandwith requirements.