To start oid from -1, try this statement:
CREATE TABLE [dbo].[main6] (
[oid] int identity(-1,1) NOT NULL,
. . .
If you want to insert -1 into an existing table, then try this:
set identity_insert main6 on
insert into main6 (oid) values (-1)
set identity_insert main6 off
select top(10) * from main6 order by oid
By the way, were the previous answers helpful?