This may not be the answer your looking for, but worth a look I guess to rule out any problems with the oracle client..
if you can connect via TNS name, which I guess you can with sqlplus.
Create a new subroutine and include the following.. (sample in VB / you can convert it to C# using online tools)
if the namespace gives you problem, make sure you have the oracle .net client driver installed.
private sub DataConnection()
Dim oraConn As String = "User Id=username;Password=passwd;Data Source=TNSname;"
Dim orac As OracleConnection = New OracleConnection(oraConn)
dim sql as string = "select something from table where ID = 2 "
Dim rsb As OracleDataReader
Dim orCommand As OracleCommand
orCommand = New OracleCommand(sql, orac)
rsb = orCommand.ExecuteReader()
' do something with the data.
orCommand = Nothing
catch ex as exception
let us know how you get on.