of course 当然，自然
That was 40 years ago,but of course you wouldn't remember it.那是40年以前的事了，你自然想不起来了。
off duty 下班
A policeman does not wear uniform while off duty.警察不上班时一般不穿制服。
on account of 因为，由于
He could not come on account of his illness.他因病不能来。
on average 通常，平均
On average babies start wakling at a year old.婴儿一般都是在一岁时开始走路的。
on hehalf of 代表，为了
He spoke on behalf of all staffs of the company.他代表公司所有员工讲话。
on board 在船上
As soon as I'm on board I always feel sick.我一到船上，就总感到恶心。
on duty 值班，上班
All but me are not on duty.除我一人外全部不上班。
on earth 究竟，到底
Where on earth have you been just now?刚才你到底上哪儿去了？
on foot 步行，徒步
She always goes home on foot.她总是步行回家。
on guard 警惕，站岗
The action of the thief soon put the policeman on guard.那小偷的行动很快使警察们警惕起来。
on purpose 故意地，有意地
It is not likely that he did it on purpose.他不可能是故意做那件事的。
on the contrary 正相反，反之
You think me idle,but on the contranry I am very busy.你认为我没有事做，但恰恰相反，我很忙。
on sale 出售，减价售出
I got this hat on sale;it was very cheap.我在大减价时买到这顶帽子，价钱很便宜。
on the one hand....on the other hand....一方面....另一方面....
On the one hand,it's cheap,but on the other hand,the quality is poor.一方面，它很便宜，但从另一这方面来说，它质量很差。
on the spot 当场，在现场
The news of important events is often broadcast on the spot over television.重要新闻经常是通过电视在现场播出。
on the whole 总的看来，大体上
I think,on the whole,we had better stay at home.我认为，总的看来，我们还是待在家里为好。
on time 准时，按时，及时
You must go to school on time.你必须按时上学。
once again 再一次
The teacher asked John to read once again.老师让约翰再读一次。
once in a while 偶尔，有时
They go for a picnic in the park once in a while.他们偶尔到公园去野餐。
once more 再一次，又一次
Tom's back home once more.汤姆再一次回家了。
once upon a time 从前
Once upon a time there was a poor wood cutter who lived in the forest.从前树林里住着一个贫穷的樵夫。
one another 互相
The members of the team support one another.球队成员互相支持。
or else 要不然，否则
Hurry up or else you'll be late.快点，否则你要迟到了。
or so 大约，左右
I shall return in a week or so.我大约一星期后就回来。
other than 除了....之外
No person other than a cook will be fit.除了厨师之外没有人是合适的。
out of 缺乏，用完；离开
He rushed out of the house.他从房子里奔了出来。
out of breath 上气不接下气，气喘吁吁
The little boy ran out of breath.小男孩跑得上气不接下气。
out of order 混乱，发生故障
My watch is out of order. What time is it?我的表出了毛病。几点钟了？
out of sight 在看不见的地方
They waved to one another till the train was out of sight.他们互相挥手直到火车消失在远方。
out of the question 毫无可能的，办不到的。
In the absence of political independence,economic independence is out of the question.没有政治上的独立，就谈不上经济上的独立。
over and over again 反复地，再三地
His father warned him over and over again.他的父亲三番五次地警告他。
owing to 由于，因为
Owing to his laziness, he failed in the test.由于他的懒惰，他没有通过这次考试。
