How do I use a stored procedure in SQL Server 2014 to call a external XML source?
EXEC sys.sp_OAGetProperty @obj, 'responseText', @response OUT 

sometimes doesn't return the output parameter.  Particularly if the response XML is longer than your @response variable.

Try:

declare  @result table(data text)

INSERT into @result (Data) exec sp_OAGetProperty @obj, 'responseText'

select * from @result