When you open a MIDI clip in Ableton Live, you can create MIDI CC envelopes. There are 128 different CCs per MIDI channel, from 0 - 127. However, Ableton Live only allows you to automate from CC 0 to CC 119, and doesn't allow clip automation to be written to CC 120 to CC127.
VOPM is an OPM VST instrument based on FM synthesis. Here is a list of MIDI CCs that VOPM will respond to:
Note the last three, all of which fall outside of the range of 0 - 119. These cannot be automated using Live's clip envelope. I've made a simple Max for Live device that gets around this issue. Download here: http://milkcrate.com.au/_other/downloads/M4L/little-scale.VOPM_extend.amxd.