Efficent way to Query a DB and write to text file - Large Recordset
You can simplify the code greatly without loosing any performance, more likely gaining instead.

There is no need to use StringBuilder - you can write directly to the StreamWriter - it will be faster since there will be no need of copy the whole data twice.

Next, there is no need to use MemoryStream - you are using it to buffer data before writing it to the disk. The same can be achieved by specifying the buffer size when creating FileStream. You should play around with the buffer size to see what is fastest on your environment - usually 4KB is used, in the sample below it is 1MB, but you have to try it on your hardware.

You should also play around with the options how to open FileStream. You should try what impact Asynchronous and WriteThrough has on your solution.

Of course, as the other answer proposed, separating reading from database and writing to the file in two threads might work as well. Try and measure three separate scenarios - just reading from the database (measure separately the query (ExecuteReader() and first Read() and then reading the other rows - because it might be that most of the time is spending to initially execute the query, not to iterate through the data), just writing to the file (dummy data) and then combined. This way you will see if there is actual reason to put the two operations in parallel.

Dim conn As New OracleConnection()
...
Sub DBExecuteQueryWriteToFile(ByVal SQLCommand As String, ByVal FileName As String, Optional ByVal Delimiter As String = ",")

    Dim cmd = New OracleCommand(SQLCommand, conn)
    Dim bufferSize = 1024*1024; // 1Mb

    If File.Exists(FileName) Then
        File.Delete(FileName)
    End If

    Using FileObject As New FileStream(FileName, FileMode.OpenOrCreate, FileAccess.Write, FileShare.None, bufferSize)
    Using StreamWriterObj As New StreamWriter(FileObject)
    Using Reader As OracleDataReader = cmd.ExecuteReader()
        Dim FieldCount As Integer = Reader.FieldCount - 1

        Do While Reader.Read()
            StreamWriterObj.Write(Reader.Item(0))
            For i = 1 To FieldCount
                StreamWriterObj.Write(Delimiter)
                StreamWriterObj.Write(Reader.Item(i))
            Next
            StreamWriterObj.WriteLine();
        Loop

    End Using
    End Using
    End Using
End Sub


P.S. few other issues in your code:


StringBuilder().ToString().ToCharArray() - no need to convert to char array.
new StringBuilder() - instead of reusing the memory already allocated to the existing builder, you are throwing that away and creating new one.
MStream.Seek() - you are seeking to the beginning of the stream but are not setting Length to 0. So if the second block written there is smaller than first one, junk data will be written to the file.