No OCZ have never used Seasonic and most likely never will as they recently bought PC P&C so they now have their own plant for making PSUs. Both companies have used Channel Well in the past though, OCZ had two models produced by Channel Well (ModXstream Pro 400 CWT and StealthXstream 400 CWT) while Corsair have had 13 SKUs made by them (CX430/500/600, Gaming Series GS600/700/800, DSG Platform HX750/850 TX950, PSH platform TX750/850 VX950 and HX1000).
Tom's Hardware published an article call
Who's who in Power 2011, which might be a little out of date now but covers so most of the major brands and shows how companies like Jeantech have no tecnhnical input into the Jeantech branded products while others like Corsair and Thermaltake design their PSUs and then outsource the manufacturing. Enermax and Seasonic design and build their PSUs.