Shivopanishad and Shaiva Upanishads

The term Shivopanishad (Sivopanisad) is met with at least twice within the framework of Shaivism. In the first instance, it refers to a large, yet untranslated, text with the proper name Shivopanishad. This texts forms a part of the so-called Shivadharma corpus, which includes the Shivadharma Shastra and Shivadharmottara. The latter is sometimes subsumed, at least in name, within the Shaivagamas as a subsidiary (upagama) of the Sharvokta Agama. This Shivopanishad is an extensive treatise on Shaivite theology and manual on Linga worship, likely follows the first two texts in the corpus in chronology, and can probably dated to the 8th century CE (?). 

A second use of the term Shivopanishad relates to the celebrated Shiva Sutras of Vasugupta generally dated to the 9th century CE. Technically, every upanishad is a secret teaching. As the Shiva Sutras were a secret teaching passed from Lord Shiva to Vasugupta, they were termed the Shivopanishad Samgraha or a collection of the secret teachings of Shiva. Just as the Brahma Sutras (a.k.a., Vedanta or Sharirika Sutras) extract and systemize the philosophy of the principal Vedic upanishads, the Shiva Sutras or Shivopanishad Samgraha extract and systemize the philosophy of the large body of Shaiva Agamas that precede it. 

And then there are the Shaiva upanishads connected to the Vedas. According to the Muktika canon, there are 14 texts considered Shaiva upanishads. This list includes the Kaivalya, Rudra Hrdaya, Atharvashira, etc. The oldest of these Shaiva upanishads belong to the middle of the first millennium BCE and considered likely composed by Pashupata ascetics prior to the advent of the Pashupata Sutras (ca. 1st century CE). These 14 are by no means the only upanishads to call the Divine by the term Shiva. Among others, the famed Shvetashvatara Upanishad is also a Rudra-Shiva oriented text and we may considered it a principal Shaiva upanishad of the Krsna Yajurveda even if the Muktika canon does not. 

Shiva alone is Guru; Shiva alone is the Vedas; Shiva alone is Lord; Shiva alone am I; 

Shiva alone is all. There is none other than Shiva.

-Varaha Upanishad of the Krsna Yajurveda, V:39

Finally, there are parts of the Vedic Samhitas that are not technically upanishads, but because of their profundity sometimes called as such. The Nilarudra Sukta of the Atharvaveda (Paippalada recension) is one such example and is elevated because it reiterates some of the potent verses of the Rudra Adhyaya (a.k.a., Sri Rudram) of the Yajurveda. Another notable example is the Shiva Sankalpa Sukta of the Shukla Yajurveda, which draws from mantras from the Rigveda, and is considered a very powerful and sublime upanishad. Using the same logic, the Rudra Adhyaya (Sri Rudram) of the Shukla and Krsna Yajurveda, which also draws from mantras of the Rigveda, may also be considered an veritable upanishad, and if so done would be the oldest of all!

All Glories to Lord Shiva, the Primal Guru and Source of all scriptures and teachings! 

OM Namah Shivaya.