Games like Observer at the hill
Games like Observer at the hill include Outlast, Firewatch, MiSide, Layers of Fear (2016), and What Remains of Edith Finch. We matched them by the tags players give Observer at the hill on Steam (Adventure, Psychological Horror, Atmospheric, First-Person, and Singleplayer): closest matches first, and among equally close ones the better-known games.

Outlast
Shared: First-Person · Atmospheric · Psychological Horror

Firewatch
Shared: Adventure · Atmospheric · First-Person

MiSide
Shared: Psychological Horror · First-Person · Singleplayer

Layers of Fear (2016)
Shared: Psychological Horror · Atmospheric · First-Person

What Remains of Edith Finch
Shared: Atmospheric · Adventure · First-Person

Poppy Playtime
Shared: Singleplayer · First-Person · Psychological Horror

Little Nightmares
Shared: Adventure · Atmospheric · Singleplayer

Little Nightmares II
Shared: Adventure · Singleplayer · Atmospheric

A Story About My Uncle
Shared: Adventure · First-Person · Singleplayer

Blair Witch
Shared: Adventure · Psychological Horror · First-Person

Outlast 2
Shared: Psychological Horror · First-Person · Singleplayer

We Happy Few
Shared: Adventure · First-Person · Psychological Horror

Penumbra Overture
Shared: Adventure · First-Person · Atmospheric

ROUTINE
Shared: Psychological Horror · Atmospheric · Singleplayer

The Beginner's Guide
Shared: Atmospheric · First-Person · Indie

The Beast Inside
Shared: Adventure · First-Person · Psychological Horror

Visage
Shared: Psychological Horror · First-Person · Adventure

No, I'm not a Human
Shared: Psychological Horror · Singleplayer · Indie

SOMA
Shared: Psychological Horror · Atmospheric · First-Person

IMSCARED
Shared: Psychological Horror · Adventure · Indie

The Long Dark
Shared: Atmospheric · Adventure · Singleplayer

Among the Sleep - Enhanced Edition
Shared: Adventure · Atmospheric · Indie

Penumbra: Black Plague Gold Edition
Shared: Adventure · Psychological Horror · First-Person

Amnesia: The Dark Descent
Shared: Psychological Horror · Atmospheric · First-Person