oxa- or  (before a vowel) ox-
combining form
indicating that a chemical compound contains oxygen, used esp to denote that a heterocyclic compound is derived from a specified compound by replacement of a carbon atom with an oxygen atom
⇒ 
oxazine
oxa-Chemistry
a prefix meaning 'oxygen when it replaces carbon'.
1928 Jrnl. Amer. Chem. Soc. L. 3075In order to avoid confusion with the ordinary meanings of oxy-, thio-, azo-, etc., it is recommended that the forms oxa-, thia-, aza-, etc., be employed to indicate the presence of hetero atoms in a ring (the a being dropped before a vowel). 
1971 Nomencl. Org. Chem. (I.U.P.A.C.) (ed. 3) B. 53 (table)Element Oxygen..Prefix Oxa. 
ORIGIN: from ox(y- 
oxa-
combining form 
or ox-
Etymology: International Scientific Vocabulary, from ox- (I) + -a- 
 : containing oxygen in place of carbon or regarded as in place of carbon usually in place of the methylene group −CH2− 
< 10H-9-oxaanthracene >
< oxazacycloheptane >
— compare aza-, thia-
or ox-
< 10H-9-oxaanthracene >
< oxazacycloheptane >
— compare aza-, thia-