| The Old Persian writing system is based off of earlier Cuneiform. It was used extensively in royal inscriptions of the Achaemenid Empire from 600 to 300 BCE. Much of the content revolves around Zoroastrian concepts. It was deciphered through the use of a trilingual monument in Persian, Median, and Babylonian to the king Darius I at Behistun. Its origins are still debated, with some placing the invention of Old Persian early in the reign of Darius I while others push back toward the time of the empire's founder, Cyrus II. The first unambiguous attestments to Old Persian only begin with Darius I's reign, and the greatest wealth of information on the function of the writing system and the language it described is gathered from these texts.
The writing system is basically alphabetic, with some syllabic qualities. There are three vowels, a, i, and u, which express both lengthened and shorten versions. The characters for the semivowels v and y commonly functioned to lengthen u and i, respectively. Consonants are divided into two types, those whose forms are independent of the vowels that follow them, and those whose forms change depending on the vowel following them. When shape is governed by a following vowel, the constant has at most three forms. Even though the following vowel is inherent in the consonant, the vowel letter is still written. This allows the expression of diphthongs without writing more than one vowel; for example 'dai' would be written with the a-form of 'd' and the vowel 'i', dai. Certain consonants, t, n, and r have forms which match up with both 'a' and 'i', meaning that tai and ti would both be written the same way, 'ta-ii'.
Beyond alphabetic characters, Old Persian contained a few logograms for concepts such as king, land, god, earth, and divine name. Nasals were not written in syllabel final position. Even though in the spoken language it is known they were not of long quality, the vowels u and i in syllable final position were always written with an extra semivowel. The written language also contained representations for numerals 1-27, 40, 60, and 120; with compounds thereof representing the rest of the numeral system.
http://www.avesta.org/op/op.htm - Avesta: Old Persian Texts
Daniels, Peter T., Bright, William. The World's Writing Systems. Oxford: Oxford University Press, 1996.
http://www.israelite.info/Source_Documents/MainPageSourceDocuments_Folder/Persian.htm - Darius Inscription at Behistun | 
