It's a cute little thing, but hardly exciting.
The park opens at 9:30, with varied closing times throughout the year. Admission also varies depending on the options you choose, starting at 12,000 won ($12) for adults. You could take a costly taxi, but buses 115, 301, 311, 314, 315, and 318 all go to O-World from Daejeon City.
Don't be fooled by the bus stop that is actually "before O-World" - you want the one next to the parking lot (the first picture shows you the view near the "before O-World" stop - the buses will turn in toward the park here). I'm not sure if all buses approach from the same direction, but it's a long walk uphill if you get off early.
Note that Daejeon has two KTX terminals depending on which line you take from Seoul; the main line station that serves Busan is much easier to handle with buses; I took a different bus back to the other station to head south to Gwangju and the stop was a long way from the station; I consider myself very lucky that I got off at the right stop and managed to find the station.