Oh that I could see him again.
这里could是表达强烈愿望的意思吗? 还有that 句子成分是什么?
这是省略句,that为连词引导名词性从句,主句省略了。这种使用方法主要用于文学作品中,表示惊讶、悲伤、愤怒、或期望等感情色彩,that从句要用虚拟语气,即从句用情态动词的过去形式。比如《韦氏高阶》在连词项下对that这种使用方法的讲解和例句:
5 literary — used to introduce a clause expressing surprise, sorrow, anger, desire, etc.
Oh, that he would come back! [=I wish that he would come back]
That it should come to this! [=I am very shocked, disappointed, etc., by this result]