Some Pashtuns of Afghanistan (particularly the diaspora) get offended when Uzbeks, Tajiks and Hazaras refer to them as Awghan (اوغان), thinking it is a derogatory distortion of the word Afghan.
But there is plenty of evidence to suggest that it is Awghan which is original while Afghan might be its Arabic or Persian modification of later times.
In ancient Bactrian documents, Pashtuns are mentioned as “αβαγανο” i.e., Abagano or Awagano (since “b” is interchangeable with “w”).
|Source: “Bactrian Documents from Northern Afghanistan II: Letters and Buddist Texts”, by Nicholas Sims-Williams, p-144
Indian astronomer Varāha Mihira mentioned them as Avagāṇa in his Bṛhat-saṃhitā in 6th century A.D.
Hsuen Tsang, in 629 A.D, refers to the people living between Bannu and Ghazni as ” O-Po-Kien”. According to Alexander Cunningham, O-Po-Kien is rendering of Avaghan in Chinese.
The original spelling Awghan occasionally occur in the medieval Persian sources of the last millennium. From Tarikh Nama-i-Herat (written in circa 1320 A.D) :-