Oasis of Fun is a relatively easy scenario and the following is a suggestion for beating it.
Firstly, build two or three flat rides on the small level area on the right side of the entrance (from an entering perspective), e.g. a Zipper, (a Ferris Wheel) and a Dinosaur-go-around, or something similar. Place down one or two ATM's, an Information Stall and First Aid around your park. Then, raise the pool's price to 3,50.
Use the leftover starting money and the revenue of the existing rides to expand the main pool. It's advisable to add a small secondary pool for a slide. The best option would be a normal Body Slide. Don't forget about the required 3,50 excitement score slide for the Tycoon level goal. After this, you the pool price may be raised to 4,00 (or 4,50 as a risky option). To complete Apprentice level, build a medium-sized junior coaster on the edge of the big lake.
Another source of money that is pretty cheap is the Giant Slide. Make one of medium height and charge a ~3,00 at lowest. On the big flat in the southwest corner (again seen from the entering perspective of the entrance), it's possible build a big coaster after removing the small palace. Add a boat transport from the previously mentioned junior coaster towards this one via the big lake. This should sum up to the needed 40k value.
I personally opted for also deleting the big palace. I kept 2 of the 8 towers and built a wooden coaster with them as a feature. It's a pain to delete the palace, but it's surely worth it.