This behaviour is determined by the compose mappings. You can find your default mappings by looking up your locale in
/usr/share/X11/locale/compose.dir, then checking the corresponding file, probably
To override a compose sequence, create a file named
as its first line (this includes the default mappings), then add your own after that:
<dead_acute> <o> : "ɔ"
You can verify your changes by starting a new
xterm: it will reflect the settings on startup. Other X programs might not; for example I’ve seen GNOME Terminal pick changes up, but not systematically, even when restarting it.