Casey, needing a par to win, produced an outstanding approach at the 72nd hole after finding the fairway bunker and two putted to win by one over Oosthuisen and Kokrak, Kokrak failing to get up and down for par at the last to lose the opportunity to win his first PGA Tour title.
Bubba Watson and Sung Jae Im tied for 4th another shot back.
Understandably Casey has developed a love affair with the course and the event and was excited to defend a title for the first time in his professional career.
“This feels very different from last year, it feels better, I'm enjoying this one so much more, said the Englishman. “Today it was a struggle, but all week has been so cool to be defending champion.
“When Tracy West and the great organizers said, what do you want on the back of your bib, and I went for the champ, because I am, and I thought, let's just roll with it, let's just stand up there proudly and do it.
“It's been so much fun, the crowds here they have been cheering both myself and Dustin on all day and everybody else in the field, but it's proud to be anybody's champion, the Valspar Championship is a great, great golf event and the people around here who support it know what I'm talking about. So to be their champion again I feel very proud and honoured.
“Anybody that plays that under par for the week is a magician. So make hay while the sun's shining and we did that. I mean, what a golf course, a great field and 8-under is the winning score, 10-under I think was the winning score last year. It's not like it's tricked up, it shows what a good golf course and some firm greens will do.”
Curtis Luck and Matt Jones lead the Australians in an encouraging 13th position although both will be disappointed they were unable to capitalise of 54 hole positions which had given them a chance to contend.
Luck’s 73 and Jones’ 72 had them recording their best finishes of the season although in the case of Jones it matched his 13th place finish at Torrey Pines.
The PGA Tour now play the World Match Play Dell Technologies Championship and an event in the Domincan Republic for those not eligible for the Match Play.
Australians in the Match-Play field are Jason Day, Marc Leishman and Cameron Smith. Adam Scott, although eligible, will sit out the event as he looks ahead to Augusta National now just over two weeks away.
Want video tips delivered straight to your inbox? Subscribe to iseekgolf.com newsletters.