I think you got the wrong link. That link looks like its something for a different game (and not an ovl file from Planet Coaster)
That said, it looks like Planet Coaster is from the same people who did Rollercoaster Tycoon 3 and I bet the .ovl files in Planet Coaster are the same format as the ones in that game were. In which case, good luck trying to reverse engineer them, you will definitely need it. I wrote the custom scenery import tools for Rollercoaster Tycoon 3 back in the day and figuring out the .ovl files was a nightmare, they were some of the hardest formats I have ever done any work on (and no I dont know of anything out there that would cover the formats of RCT3 nor do I have any old stuff from back then so any reverse engineering would basically need to be done from scratch)
Sorry, I've edited the link.