Hi Arsala,
check this Oracle Support document ->
Detailed Overview of Connecting Oracle to MySQL Using DG4ODBC Database Link (Doc ID 1320645.1)
Also read the following document. I wrote it. It is for Microsoft Sql Server , but you will get the idea.. (it is also using DG4ODBC)
http://ermanarslan.blogspot.com.tr/2016/01/rdbms-oracle-to-sqlserver-dblink.html