If you try to run it directly (in SQL*Plus or some other client) - does it run?
Anyway, try to surround your alias with double quotation marks - I've tried with other language and it seems to work
Dim sql As String = "select itemid,descr as \"SOMEGREEKTEXT\", DESCRSH2,VALUE from ITEMS where ITEMID like " & "'" & TxtCode.Text & "'"
I think that to get a meaningful results your database should support Greek character set...
Not part of your question but I've noticed:
1) Unless the text you pass (TxtCode.Text) includes the "%" - it seems your query may not returned as expected...
2) This is very important when using Oracle and ODP.NET - try to use bind variables as much as possible as you may gain a big improvement in performance and database health as well has avoid many encoding and similar issues.