user: Question: What company constrcuted the vehicle with a circuit of Oulton Park on 15 April?
Columns: ['Race Name', 'Circuit', 'Date', 'Winning driver', 'Constructor', 'Report']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['II Race of Champions', 'Brands Hatch', '12 March', 'Dan Gurney', 'Eagle - Weslake', 'Report']
SQL:

assistant: SELECT "Constructor" FROM "Table" WHERE "Circuit" = 'Oulton Park' AND "Date" = '15 April';