The particle عسی is one of the verbs of رجاء. It can be used in one of two ways:
1. As a فعل تام: in this case it will be followed by a sentence which has أن. This sentence will become the فاعل.
عَسَى أَنْ يَنْفَعَنَا أَوْ نَتَّخِذَهُ وَلَدًا
2. As a فعل ناقص: in this case it will be followed by a noun. This will be the اسم of عسی. This will then be followed by a sentence which will be considered to be the خبر of عسی.
عَسَى اللَّهُ أَنْ يَعْفُوَ عَنْهُمْ