A is the member of the OECD
A is a member of the OECD
which is correct?
"A" (whoever that is) is one member, presumably of many, of OECD, therefore "a".
Unlikely though it is, if OECD only has one member, who is called "A", then you would use "A is THE (only) member of OECD" - however, I can't imagine you will ever need this!