In Grampa, a prediction made by Abrog, the High Sky Prophet of Perhaps City, is significant to the plot. We don't actually see the wording of the prophecy until near the end of the book, which I see as a narrative failure on Thompson's part. Abrog interprets his prophecy to mean that the city ruler's daughter will be married to a monster, and the monarch initially accepts it, only to place all the blame on Abrog once it's revealed to actually mean something else. Not that I don't think Abrog deserved most of the blame, since he tried to use his prediction as an excuse to marry the princess himself, not to mention his secret second life as an illegal wizard.
Our next member of the Ozian prophetic party is Abrog, the soothsayer at the court of King Cheeriobed in Giant Horse. He's an interesting character, trying to do what he can to save the Ozure Isles from the wrath of the monster Quiberon, but not doing so in the most ethical manner. At the end of the story, Cheeriobed banishes Akbad from the Sapphire City, but I still hold out hope that he redeemed himself later on. In fact, he actually does in March Laumer's Good Witch. Anyway, Akbad does seem to have some familiarity with magic, in that he carries a magic descriptionary in his pocket and looks through an old book of necromancy; but he never does any actual soothsaying within the story.
Just two books later, in Yellow Knight, we come to Chinda, the Chief Prophet and Seer of Samandra. He has a tower room that he uses for his work, and possesses a magic telescope that he uses to find things. When he successfully locates the lost Comfortable Camel, the Sultan promotes him to Magician Extraordinary and Grand Bozzywoz of the Realm. We never learn exactly what a Bozzywoz is, but one of Chinda's lines indicates that his duties include leading processions. The same book also has Hurreewurree, the Chief Counselor of Quick City, referring to a prophecy in the Book of Stars.
Early in Purple Prince, Randy and Kabumpo come across a soothsayer in Follensby Forest. In between yelling "sooth," this man directs the boy and elephant to the castle of the Red Jinn.
Perhaps the most competent prognosticator in the series is Bitty Bit, the Seer of Some Summit in the Land of Ev. This brownie-ish man lives in a castle with a shooting tower, and has the power to see the past, present, and future.
I close this post with some references from the last book in the Famous Forty, Eloise and Lauren McGraw's Merry Go Round. In the city of Roundabout, Roundelay the Sphere-Seer is quite fervent in his attempts to fulfill a short prophecy, which goes as follows:
"The ring will bring the King,
The King will bring the Thing
The treasure's found
The ring will bring the King."
The prophecy does end up being fulfilled, but not in the way Roundelay plans. In the same book, Prince Gules and Fess consult the Oracle in the Coracle, which turns out to be a crystal ball that will answer any three questions in exchange for three gold pieces. I'm not sure what a crystal ball would do with money, but maybe it has an agent or something.