大学英语：重要词汇on occasion(=now and then)不时地，必要时

478.by occasion of(=because of)由于

479.occupy oneself with(in)忙于(某事)

480.it occurs to sb. that… 某人想到…

481.once and for all=once and forever永远地;all at once(=suddenly，now)立即，马上;once in a while (=occasionally)偶尔;(just)for once 就这一次

482.(all)by oneself 独自(没有别人帮助)

483.operate on sb. 给某人做手术;come go into operation 开始运转;put bring sth. into operation 使…投产，运转

484.be of the opinion 持有…的看法

485.in one's opinion 按某人的看法

486.be opposed to… 反对…

487.be opposite to 与…相反的

488.(be)in order(=acceptable)合适的，恰当的;in order 井井有条，处于良好状态;out of order(=in bad condition)出毛病，发生故障