Initial o in Pali may represent a Vedic o or a Vedic au (see ojas, ogha, etc.). Or it may be guṇa of u (see oḷārika opakammika, etc.). But it is usually a prefix representing Vedic ava. The form in o is the regular use in old Pali; there are only two or three cases where ava for metrical or other reasons, introduced. In post-canonical Pali the form in ava is the regular one. For new formations we believe there is no exception to this rule. But the old form in o has in a few cases, survived. Though o; standing alone, is derived from ava, yet compounds with o are almost invariably older than the corresponding compounds with ava (see note on ogamana).