The o5m format is now supported
Both splitter and mkgmap now support the o5m format. You can read these files just by putting them on the command line as with any other file. For splitter the default output format remains pbf, so if you want it to produce o5m files on output then you will need to give the --output=o5m option.
Files in the o5m format are larger than those in the pbf format, but are faster to read, particularly for splitter when it has to re-read the input file several times.
There is not such a large speed advantage for mkgmap.
The o5m support was added by Gerd Petermann.