同意陈老师的解释。我补充一点关于once 和one time表示次数时， 在表示强调的情况下可以互换的情形。one time 语气更强些。

例如： I　have been there only one time. = only once.

托福听力考试答题指令：You will hear the recording for just one time.

雅思听力考试答题指令：The recording will be played once only.

【1】once 用法如下

(1)adv. 一次，一回;曾，曾经，从前，昔日

I have been there once.   
我到过那儿一次。

I once went around the world.   
过去我周游过全球。

(2)conj. 一…就 ，一旦…就… ，一经...便;当…时候，每当

Once you begin, you must continue.   
一旦开了头,你就必须继续下去。

Once you have taken the examination, you'll be able to relax.   
你一旦考过试,就可以轻松一下了。

Once you have promised you must do it.   
你既已答应,就应该去做。

(3)n. 一次，一回，一度

Once is enough for me.   
我一次就够了。

She's only done it the once so don't be too angry.   
她只做过这一次,所以不要太生气。

【2】one time一度, 某时;一次

My brain can't hold so much information at one time.
我脑子一下子记不住这麽多信息。

He was at one time so devoted to piano playing.
他曾经那么沉迷于弹钢琴。

At one time, she is fine, but at another, she is abnormal.
有时候，她表现得很好，有时候又失常。

It's a physical impossibility to be in two places at one time.
同时在两地是自然规律上不可能的事。

The census is taken one time every four years in our country.
我国每四年一次人口普查。

He has at one time or another argued both for and against most of the changes in the city over the past30 years.
他对这个城市近30年来所发生的大部分变化时而表示认可，时而又反对。

At one time I used to go skiing every winter.
我有一度每到冬季就去滑雪。

【3】at one time 曾经； 同时； 一度

My brain can't hold so much information at one time.
我脑子一下子记不住这麽多信息。

At one time, she is fine, but at another, she is abnormal.
有时候，她表现得很好，有时候又失常。

It's a physical impossibility to be in two places at one time.
同时在两地是自然规律上不可能的事。

The two continents were connected by a land mass at one time.
这两个大陆曾经由一大片陆地连接着。

An amount of food taken into the mouth at one time; a mouthful.
一口食物; 一口，少量一次吞入口的食物量; 一口的量

All the copies of a publication printed at one time from the same set of type.
印数，印次用同一个版本一次印刷的所有印刷物

He was at one time so devoted to piano playing.
他曾经那么沉迷于弹钢琴。