Follow the yellow highway line beyond the golden arches of Grand Rapids and the tulip fields of Holland to Rainbow Lake and Misty Carousel Mountain, where Castle Park hides from the world by vanishing from the road map. Old U.S. Post Office 4922 is the only official municipal mark in this tiny town of 300 with no city hall, police station or even a school. It boasts instead an 1890 Prussian castle where the 20th Annual International Wizard of Oz Convention met this weekend. At the castle entranceway, the sign cautions:

Drive slowly - children everywhere.

Eighty earnest ozmotologists, ages 7 to 61, flocked from Munchkindland (the East Coast), Gillikin (Northern U.S. and Canada), Quadling (the South) and Winkie Land (West Coast) to celebrate the nonsensical tales of Oz author L. Frank Baum, who wrote many of the known as The Sign of the Goose before it mystreriously disappeared (was it by flood or fire?) along Lake Michigan:

"L. Frank Baum was a character," recalls poet Eunice Tietjens. "He could honestly not tell the difference between what he had done and what he had imagined. Everything he said had to be taken with at least a half pund of salt."

Baum's disciples, a rare collection of literary people, psychologists and mathematicans, give the same impression from time to time, espicially when they tell tales about past conventions. At the 1964 meeting for example, Fred M. Meyers, 51, Oz excutive secretary, swears that a cyclone swooped down in Bass Lake, Ind., and blew the roof off a house, carrying away a scarecrow wearing the pajamas of Harry Neal Baum, Frank's son.

Meyer claims it is all quite true. And he is a fountain of Oz knowledge like most of the 1,600 national club members. The club has increased by nearly 100 each year since it was founded in 1957 by Oz collector Justice Schiller, who was 13 at the time.

Oz scholars pride themselves on knowing that Baum named his magic place label. Competend ozmotoligists can trcite all seven Baum pseudonymes, the title of all 14 Baum Oz books, the 26 stories by Royal Oz Historian Ruth Plumly Thompson, as well as the titles of Baum's plays and motion pictures first premiered in Grand Rapids at the turn of the century.

Club members point to comtemporary Oz influences on "Star Wars" and have a serious missionary zeal to turn the tide of Oz controversy and win literary respectability.

The books were banned in New York City during the McCarthy era and were prohibited in the Detroit Public Library until 1973 because library director Ray Ulveling judged the tales to be "of no value, encouraging nagativism and misleading young minds to accept a cowardly approach ti life."

It was not until Columbia University held an Oz exhibition in 1956 featuring the Schiller collection that Oz received formal distinction as being among the greatest American fairy-tales.

Oz fans prepare for the intertional convention all year by competing in the Oa memorabilla auction which sets prices around the country. They rehearse for Oz puppet shows, singlatong and the prestigious trivia examination.

In the log cabin drawing room filled with Oz banners here, Gilliken Erick Neher, 12, of Dekalb, III., was victorious in the junior league trivia test covering the first six books. The toughest question was: How long do Mangaboos live? It's four to six years, Erick explained: "Mangaboos are a type of people that grow from bushes in 'Dorothy and the Wizard of Oz.' "Erick says he started reading Oz stories when he was "a little kid." His favorite character is H.M. (Highly Magnified) Woggle-Bug (Thoroughly Educated.)

'Woggle-Bug is different from the others who are all-evil" he said. "But Woggle-Bug is in between and gets a little obnoxious because he is smart and can see it. It guess he's more than humans," Erick explained.

The adult trivia champ was Munchkin Ed Brody, 15, of Green Brook, N.J., who engaged in a tough game of Ozcapades (a cross between checkers and tic-tac-toe) with Suzanne Van-Camp, 14 of Northbrook, III., whose CPA father, John, is Oz Club traesurer.

Brody only missed four questions out of 54 the toughest being: What country was Orin first princess of?

"The answer is Gilkenny. That's as close as you can get to absolute trivia," said Brody, sporting an Oz T-shirt studded with magic transformation word: PYRZQXGL (pronunciation unknown - from "The Magic of Oz," 1919).

Brody won his trivia title from last year's champ, Laura Jane Musser, 61, who sat in a nearby corner reading expert present, Mrs, Musser is a "The Wizard of Oz." The oldest Oz Weyerhauser Lumber Co. heiress who boasts friend of L. Frank Baum.

She joined the Oz Club in 1972, and confessed it was a little pretentious back then - "Very stuffy and formal indeed."

Mrs. Musser owns all 14 Baum books including seven first editions worth about $3,000 although she says their value is hard to estimate.

One hundred fifty items from the world of Oz were auctioned off by Dick Martin, 50, who kept the eager ozmatologists surprisingly calm with his soft-spoken admonitions.

Martin, an Oz book illustrator, looked like the thin man as he cradled each bit of Oz lore affectionately in his arms. He noted that July Garland's Ruby slippers (really silver in the orginal text) brought the highest price at any Oz auction on record, going for $15,000.

Martin offered "a gorgeous hand-made Oz branding iron" (sold for $7), 38-42 ($17.50), an Oz peanut butter glass ($2), a 1977 TV Guide describing the 1937 MGM movie ($1.05), and a rare German edition of Alexander Volko's Russian version of Oz ($30).

This Oz craziness may be lost on the casual observer. But Munchkin Barbara Koelle, 52, of Philadelphia offers some explanation in her recent essay printed in "the Bomb Bugle, a journal of Oz" published Spring, Autumn and Winter. Mrs. Koelle believes the collectors bug is responsible, and that many Oz collectors are already into things like miniature trains and Tarzan. "Then I suppose there is the wheeler-dealer instinct. And you make good friends around the country," explains Mrs. Koelle, who received the 1977 L. Frank Baum Memorial Award for distinguished Oz service.

Oz scholars don't spend too much time analyzing the phenomenon or indulging in Freudian interpretations which, for example, see the Wicked Witch of the West as Dorothy's evil mother, or see the lack of a brain, heart and courage in the Scarecrow, Tin Man and Cowardly Lion as suggesting that Baum had a castration complex.

These approaches are shruffed off. Most Baum followers believe his own life experience. His distaste for the bungling military, for instance, acquired during an undistinguished stint at Peekskill Academy. Baum's fierce femael armies may have been inspired by the suffragette notion of his mother-in-law, Matilda Josylin Gage.

Perhaps the Tin Man's wish for a heart came from Baum's own heart condition, which slowed him down but never dampened his spirit of advanture. He was a jack of all trades in his time: a Shakespearean actor, newspaper editor, traveling salesman and song writer, and was bankrupt time and again.

But he loved to write for children, composing fantasies in his prize-winning garden while puffing on a big cigar. At his summer home near the castle, he once wrote:

"To write fairy stories for children, to amuse them, to divert restless children, sick children, to keep them out of mischief on rainy days, seems of greater-uo novels. Few of the popular novels last the year out . . . whereas a child's book . . . is the same always, since children are always the same kind of little folks with the same needs to satidfy."

Baum died in 1919 at the age of 63, and the international Wizard of Oz club now seeks to satisfy those needs for the modest annual fee of $5.

The next munchkin convention is July 9 at Haddon Heights, N.J., and there will be a Winkle Covention July 22-24 in Yosemite National Park, accordi*ng to Fred M. Meyer, P.O. Box 95, Kinderhook, Ill. 62345.