over.
原因是原句中用的是have been doing,而不是have done 句型.
Have been doing 是现在完成进行时,强调在一段时间内都在做某事.
所以这个句子有2种填法,
1.He has been studying Chinese for over one year
2.He has studied Chinese for only one year
PS:
另一个角度理解,have been doing 意思是一直在坐某事,强调时间较长,比如
He has been playing PC games all day!
他一整天都在玩电脑.
所以如果前面用have been doing 后面却用了表示仅仅的 only,前后语气就矛盾了.