Opening Oracle OleDb connection succeeds, while managed driver connection fails

From C# I'm connecting to Oracle using OleDb doing the following:

String connectionString = "Provider=OraOLEDB.Oracle.1;Persist Security Info=False;User ID=x;Password=y;Data Source=z";

var connection = new System.Data.OleDbConnection(connectionString);
connection.Open();

That works fine.

From the same process, I'm trying to connect using the managed driver doing the following:

String connectionString = "USER ID=x;PASSWORD=y;DATA SOURCE=z";

var connection = new Oracle.ManagedDataAccess.Client.OracleConnection(connectionString);
connection.Open();

That fails with the following error:

Oracle.ManagedDataAccess.Client.OracleException (0x80004005): ORA-12154: TNS:could not resolve the connect identifier specified ---> OracleInternal.Network.NetworkException (0x00002F7A): ORA-12154: TNS:could not resolve the connect identifier specified
   at OracleInternal.Network.AddressResolution..ctor(String TNSAlias, String instanceName)
   at OracleInternal.Network.OracleCommunication.DoConnect(String tnsDescriptor)
   at OracleInternal.ServiceObjects.OracleConnectionImpl.Connect(ConnectionString cs, Boolean bOpenEndUserSession, String instanceName)

Server stack trace: 
   at OracleInternal.ConnectionPool.PoolManager`3.Get(ConnectionString csWithDiffOrNewPwd, Boolean bGetForApp, String affinityInstanceName, Boolean bForceMatch)
   at OracleInternal.ConnectionPool.OraclePoolManager.Get(ConnectionString csWithNewPassword, Boolean bGetForApp, String affinityInstanceName, Boolean bForceMatch)
   at OracleInternal.ConnectionPool.OracleConnectionDispenser`3.Get(ConnectionString cs, PM conPM, ConnectionString pmCS, SecureString securedPassword, SecureString securedProxyPassword)
   at Oracle.ManagedDataAccess.Client.OracleConnection.Open()

I've received errors like this in the past when there was a problem with tnsnames, but I've never had a situation where OleDb works when the managed driver doesn't.

I've double checked and the user, password, and datasource are identical in the two connection strings.

Sounds like your TNSNames isnt setup correctly. You could also do a connection string like this to get around using tnsnames

static string constr = @"Data Source=(DESCRIPTION=
         (ADDRESS_LIST=(ADDRESS=(PROTOCOL=TCP)(HOST=yourhostname )(PORT=1521)))
         (CONNECT_DATA=(SERVER=DEDICATED)(SERVICE_NAME=XE)));
        User Id=system ;Password=yourpasswrd"; 

this is error ORA-12154: TNS:could not resolve the connect identifier specified?