I don't think there's any doubt that osier is not really a common word ... except in Chi ... up until now.
When I first started noticing the word in Chi, I thought it might be some kind of obsolete occupation, like a hosier perhaps. But it seems it's a type of willow sometimes used for basket-making.
Its most recent appearance in the puzzle was in yesterday's Challenge (REPULSION), when it got a respectable 123 hits. But, as I've indicated before, I don't want a word to be common purely because a lot of players know it because it's always been common. The word will be rare from now on.