The surest way to find oil is to raise a derrick and dig a hole in the middle of a hundred other oil wells, which is how the high-dollar horse outfits win the Kentucky Derby. They pony up a $250,000 stud fee, sweet-talk their half-million-dollar mare into the blind date and then, as Bull Hancock once said, "Pray like hell."

So when Sunny's Halo lit up this gray and stormy day with a rainbow by winning the 109th Kentucky Derby, it was a wildcatter's improbable oil strike in unexplored territory. This Derby winner came from a mama and daddy who did their stuff for $7,500. It didn't come from a Bluegrass castle but from a farm on the eastern shore of Maryland. Anyone thinking such a poor child could win the ritzy Derby also might try the Indianapolis 500 in a clunkety-clunk station wagon.

Only five months ago, the horse needed crutches to get from bed to the breakfast table. He had bad ankles and bad shins. When the other guys were working on their resumes for the Derby, Sunny's Halo was off at Hollywood Park--swimming. The idea was that such therapy for two months would keep him strong while allowing his sore tootsies relief from the pounding of running.

Perhaps there breathed a blueblooded Bluegrass hardboot who believed Sunny's Halo had a chance to win this Derby, because, after all, his mama's family is full of Northern Dancer relatives (very hoity-toity) and his daddy has done so well that his boudoir manner now brings $30,000 a night. But if such a hardboot exists, he wrote off Sunny's Halo as a foolish gamble because he had run only two races this year before the Derby.

Sacrilege, that.

Was this bargain-basement commoner so brash as to think he could crash the Derby party without an engraved invitation earned with the obligatory six or seven Derby prep races?

But the Arkansas Derby victory by Sunny's Halo three weeks ago was testimony he was fit, and, hoity-toity or not, the 134,444 customers (Jimmy Carter, Gerald Ford and George Bush dropped by) made him the second choice behind Marfa. Marfa, a $300,000 colt, finished fifth.

Eddie Delahoussaye quickly moved Sunny's Halo to the front end of a 20-horse field. With three-quarters of a mile left, Delahoussaye, who steered Gato del Sol through traffic to win last year's Derby, had Sunny's Halo running in first. They lost the lead going into the turn, but Delahoussaye was unworried.

"When we were running so good and easy, I thought, 'We're going to be all right,' " he said.

Late moves by Desert Wine and Caveat fell far short of Sunny's Halo, who gave a convincing demonstration of the right stuff that trainer David Cross noticed long ago.

Cross is 48, a Canadian who for 25 years has been respectable without gaining fame. He trained 35 horses two years ago before he fell smitten with Sunny's Halo. He dropped 20 to concentrate on his big horse. The owners of the other 15 dropped David Cross, who said, "I don't blame them. I worked only with Sunny's Halo for 11 months, and they weren't getting what they paid for."

Cross gambled. He gambled that Sunny's Halo was his oil well. "He had desire and will, and that's the difference between a good horse and a bum."

It was a gamble all around, for the owner, David J. (Pud) Foster, 67, had about quit the game in frustration when Cross talked him into trying one more breeding of his mare, Mostly Sunny.

Foster is no Vanderbilt or Whitney. He is, according to a Toronto newspaperman/friend, "a hustler who makes money somehow." By Foster's description, he advances seed money to new companies for mining ventures. If the mine produces, he gets money back and then arranges for the big-bucks financing the company will need.

Listen, please, to Foster's story of a gamble.

"This will make you laugh. I had two mares, Mostly Sunny and Dup Dup. Dup Dup aborted twice. Then I bred Mostly Sunny to Kinsmans Hope. We sent the colt to Ocala, out of the Canadian winter, and he stepped in a hole and broke his shoulder. We had to put him down.

"Then we had the two mares in foal to Vice Regent. He's a $50,000 stud fee now, with 20 up front, but we had him for $2,500 each. Dup Dup died in foal, and Mostly Sunny aborted.

"I was ready to give Mostly Sunny away for a corned beef sandwich. I was awfully discouraged. It might not have been much money for some people, but it was for me. Then I was induced by David Cross to take another shot--this time with Halo, for $7,500.

"The Queen of England had sent a horse to Halo, and I figured if he was good enough for the Queen, he was good enough for me."

The result of the Halo-Mostly Sunny tryst at Windfields Farm at Chesapeake, Md. was Sunny's Halo, and late today Foster said, "If this one hadn't worked, I'd be out of racing."

Instead, he is headed for the Preakness with a $7,500 colt now worth maybe $15 million. They'll be celebrating in Patty C's, too. That's a restaurant outside Toronto, owned by Patty Cross, the trainer's wife. The brave patrons will raise high a drink named the Sunny's Halo.

"It's amaretto, tequila, Cuantro, a scoop of orange sherbet, orange juice, ice melted in the blender--rimmed with cinnamon and sugar," she said.

Talk about gambles.