Found this place through Maps and decided to visit the same day.
The park is well organized and wasn't crowded at all during our stay. This resulted in a super enjoyable experience, as we were able to go through the slides non-stop with little to no wait in-between the rides.
The pools are not too deep, and that adds up to the safety.
All slides and play areas are kids-friendly, so the little ones can enjoy the whole park without missing out on anything.
The premises were clean and admission ticket was reasonably priced.
From 5 till 7 PM, admission is just 6 EUR, which makes it extremely good value in sunny summer days.
The only remark I have is about hair driers: as mentioned in other reviews, these are not too many and definitely not within kids' reach, which is surprising for a place like this.
Overall, we enjoyed our time at Oplà and we will surely be back.
It might not be as big or slides-packed as other nearby water parks, but it was also not as crowded.