We have answered to you at our forum: http://forums.devart.com/viewtopic.php?f=1&t=29549
As a workaround, you can try using:
1) the OCI mode;
or
2) this code:
OracleConnection conn = new OracleConnection("User Id=user;Password=pass;Server=dboracle;SID=sid;Direct=true");
conn.Open();
OracleTransaction t = conn.BeginTransaction();
OracleCommand comm = new OracleCommand(@"MERGE INTO ...");// place here your merge statement
comm.Transaction = t;
comm.Connection = conn;
comm.ExecuteNonQuery();
t.Commit();