Shiva Purana
प्रणवार्थश्शिवः साक्षात्प्राधान्येन प्रकीर्त्तितः ॥ श्रुतिषु स्मृतिशास्त्रेषु पुराणेष्वागमेषु च ॥ ६॥
The meaning of the oṁkāra is directly described as Shiva with primacy. In the scriptures, in the scriptures of memory, in the Puranas and in the Vedas.
english translation
praNavArthazzivaH sAkSAtprAdhAnyena prakIrttitaH ॥ zrutiSu smRtizAstreSu purANeSvAgameSu ca ॥ 6॥
hk transliteration by Sanscript