if only new records should be added you should first make update to make sure you only select new rows. I would probably add new column "processed" to oracle table bit/boolean that would state if record was already saved to sql.
Then Any record that is already saved into sql should have new property processed set to true.
Also why are you storing data from datareader into textbox? if it's for temporary storage you should rather use some simple class.
And if you are expecting that there will be lot's of data(at least thousands) every cycle you should consider using bulk insert.
Well for Bulk insert i would use codes from here
Bulk Insert in .NET applications: Part 1
And because you have only view of table you can't add tag saying that it was already downloaded to sql server.
If you use SQL Server 2008 or later I woudl always add data to tmp table(it would be same table as tabe in which you want to save data) and Then use TSQL MERGE in stored procedure that could be called aso from your code or periodically by SQL Agent.