Maybe it was easier to get 'good/useful' sound out of it, than out the ARP and the EMS synths. Or the kind of sounds he was looking for. Indeed if it hadn't have been for the RMI, Oxygene would've turned out very different.
Besides, I don't so much think it's a guitaresque sound any more, it has a very similar quality to the O5's main bass.
When one cuts away the bass frequencies one is left with this: http://www.sendspace.com/file/q0zoas
A rather sawish sounding sound.
I'm guessing this is the guitaresque part of the sound you're talking about.
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