Pranava and Panchakshara

praṇava and pañcākṣara

Vidyeshvara Samhita of Shiva Purana describes praṇava (प्रणव).
AUM (ॐ) is the subtle (sūkṣma) praṇava and pañcākṣara (पञ्चाक्षर) is the gross (स्थूल) ) praṇava.
AUM (ॐ) comprises of a+u+m+nāda+biṁdu (अ+उ+म्+नाद+बिंदु).
pañcākṣara (पञ्चाक्षर), or popularly known as the Shiva Panchakshara, is the five-syllable mantra “namaḥ śivāya” {॥ नमः शिवाय॥}.

Some shlokas from Vidyeshvara Samhita of Shiva Purana regarding praṇava and pañcākṣara:

प्रः प्रपञ्चो न नास्तीति युष्माकम् प्रणवं विदुः।
प्रकर्षेण नयेद्यस्मान्मोक्षं वः प्रणवं विदुः॥

सूक्ष्ममेकाक्षरं विद्यात्स्थूलं पञ्चाक्षरं विदुः।
सूक्ष्ममव्यक्तपञ्चार्णम् सुव्यक्तार्णम् तथेतरत्॥

अकारश्च उकारश्च मकारश्च ततः परम्।
बिन्दुनादयुतं तद्धि शब्दकालकलान्वितम्॥

शिवनाम नमः पूर्वं चतुर्थ्यां पञ्चतत्त्वकम्।
स्थूलप्रणवरूपं हि शिवपञ्चाक्षरं द्विजाः॥

Vidyeshvara Samhita further states that the best period for pañcākṣara japa is from śukla caturdaśī to kṛśṇa caturdaśī, especially during kṛśṇa caturdaśī and the months of bhādrapada and māgha.

According to Shiva Purana, māgha kṛśṇa caturdaśī is to be observed as MahaShivaratri.