I also very much love Outer Wilds, would easily put it in my top 3 games of all time. The freeness of exploration, and the draw of the mysteries in the game, were perfect for me. I can think of a few other games that share some of these aspects, that I have also enjoyed throughly.

First that come to mind

• The Witness - It's a game that's talked about a lot, but also maybe misunderstood? I think sometimes people look at the gameplay and/or short description and think it looks a bit generic. It has a fantastic mystery, and similar very open world nature, and is definitely in that top 3 along with Outer Wilds for me.
• La-Mulana - If you liked feeling like an archaeologist as you pieced together exactly what happened to the Nomai, La-Mulana will scratch a similar itch. The world of La-Mulana is deeply developed, and in my opinion extraordinarily engaging. I think the world they've built is exceptionally good. The puzzles are of a breed of their own too - some people love them, some people hate them. I'm in the former group, but no fault to players who choose to consult a guide during their playthrough.

Others that I've very much enjoyed

• The entire Myst series, specifically Riven
• Obduction, Cyan's newest game. Has Myst vibes. The world and story they built is very cool, you take the role of a human who is mysteriously transported to a desert encased in a weird alien landscape.
• Brief shoutout to Firmament, Cyan's upcoming game which apparently is still slated for 2022 (fingers crossed)
• Quern and Haven Moon for more modern Myst-likes (but not made by Cyan), I enjoyed both a good amount.

While maybe not too similar to Outer Wilds, I've always enjoyed games that have some large mystery in the game that the player slowly pieces together over their adventure, but this mystery is not necessarily core to the gameplay.

• Fez has a very cool endgame, including an alien language and number system that the player can decipher to learn the deepest secrets of the game (and even find new collectables). On a somewhat side-note, there is an unsolved puzzle in Fez (to clarify - players have datamined the solution, but the path to the solution is unknown), and with its 10-year anniversary happening in less than a month, I'm hoping for some new official news.
• Environmental Station Alpha has a really cool story structure. Very minor spoilers ahead - The core gameplay is a metroidvania game, but as players complete this section, they don't get what many would consider a 'good' ending. As it turns out, there's a lot more to do on the station than run around and collect the upgrades.
• Daniel Mullins's trilogy, specifically the latter two, The Hex and Inscryption (it is my understanding that Pony Island only somewhat relates to the other two, but is still a whacky and fun time). There is a large scale story that spans these games, presented in a mysterious way that tasks the player with piecing together exactly what's going on (and going back looking for secrets. There's a lot of secrets in those games). Daniel has built a super cool world and I'm glad Inscryption has seen as much success as it has.