两者都可以表示：用他自己的话说,通常可以互换.但是in的方法更为普遍,在英文一些官方表达中,及词典中in one's own words 更为普遍些.
例如：
Tom can talk about the festivals in his own words.等于
Tom can talk about the festivals on his own words.
汤姆能用英语流利表达节日的相关知识.