This is the plastic cartridge HEPA filter that is installed inside of the cabinet on the Millenium 5 liter oxygen concentrators. It typically has a foam pre-filter installed on the inlet side. Refer to the illustration below for details on how it is installed. If your OPI indicator is alarming, a clogged filter could be the problem.
The inlet filter and the foam pre-filter are accessed by removing the panel cover on the back of the unit.
The inlet filter can be seen here with the foam pre-filter installed onto the top. Both filters can be easily removed by simply lifting them up off of the inlet opening.