From the ALSA wiki
This is the OssEmulation module that emulates the OSS sequencer interface for MIDI stuff. It supports both playback to your soundcard's MIDI synth device (if it has one, otherwise use Timidity) and playback to external MIDI keyboards. Recording from MIDI keyboards doesn't work yet. This is a known bug (see Alsa-devel 2002/8/26 and 2003/12/7).
This module provides the two MIDI OSS Devices:
/dev/sequencer (recording doesn't work yet) /dev/sequencer2
Please also see the OSS Sequencer Emulation page.