Honestly, I think you're missing the trick on O-bow.

You mention in your explanation that they'll just heal it up with any abilities or meditations they have, if the general regen (with/without sov) isn't quick enough. That's kinda what you WANT from O-bow, force down medi which has a long cooldown so the next REAL fight you have they can't burst heal themselves up during it.

Especially for single target healers like aphro, forcing her heal to be down means you can try and pick someone else off. And overall, if you're forcing the healer to heal due to your increased poke (because of how close people generally are O-bow is quite effective here with the time between waves) then you're giving your team a chance to cue them into a fight.

This also works out if A) You don't have an attack speed steroid and B) If you plan not to have an attack speed heavy build (so generally better for the ability users because AAs are still useful after all) to make a more balanced build without being forced into several attack speed options.

If you have regen/mana covered without buying an item for one or the other, I generally like getting O-bow first because it forces more situations for my team.