var context = new BackgroundSeContext();
var domain = context.Domain.SingleOrDefault(d => d.Id == "Ooo");

if (domain == null)
{
    domain = new Domain {Id = "Ooo"};
    var res = context.SaveChanges();
}

SELECT TOP (2) [Extent1].[Id]          AS [Id],
               [Extent1].[DisplayName] AS [DisplayName]
FROM   [dbo].[Domain] AS [Extent1]
WHERE  N'Ooo' = [Extent1].[Id]

context.Domain.Add(domain);