Lord Shiva is the same as the Pranava

हरिविधिमोहनिवारणं नाम पंचत्रिंशोऽध्यायः

Removal of the illusion/confusion of Lord Vishnu and Lord Brahma – Chapter 35

उपमन्युरुवाच |

Upamanyu said (to Lord Krishna)

शिवो हि प्रणवो ह्येष प्रणवो हि शिवः स्मृतः || 43 ||

Lord Shiva himself is the Pranava (the primordial sound Om); this
Pranava itself is called Lord Shiva.

Shloka 28 of Chapter 48 of Parvati block of Rudra Samhita says just
the same thing in slightly different words.

शिवो नादमयः सत्यं नादः शिवमयस्तथा |
उभयोरन्तरं नास्ति नादस्य च शिवस्य च || 28 ||