Onirism is a whimsical narrative adventure where you fight monsters while traversing fantasy world to reclaim your stuffed animal, Bunbun. As you explore by foot, car, and boat, you discover a world full of creatures, some friendly, others not, and upgrade and equip new weapons, abilities, and items. It's a celebration of classic run-and-gun exuberance, but the usual power-fantasy is replaced by a young girl's quest for her beloved imaginary friend.

You play Carol, who wakes up to find a strange shadowy figure emerging from a portal stealing her plushie Bunbun. Not content to sit by, you follow them back through the portal on a quest to reclaim Bunbun from the dark forces and give the thief "a good telling-off". The world you discover is full of larger-than-life plants and glowing and magical flora, fauna, and crystals. As you explore, you encounter hordes of enemies you must defeat (and scold) using various children's toys and household equipment turned into weapons.

Gameplay involves defeating enemies with an assortment of weapons, including a water pistol, an umbrella and a blow dryer. You also need to run and jump across pitfalls and avoid obstacles and traverse the map. To cover larger distances you use a variety of vehicles like an off-road car and a powerboat. As you play, you meet some friendly locals who give you tips and can help you unlock new abilities. Sometimes, revisiting old locations with your new items and powers can uncover secrets and even extra levels. The game is constantly being updated, with new weapons, areas, secrets, game modes, and ways to play constantly being added.

The result is an intentionally bizarre combination of platforming and whimsical combat that creates a lighthearted and comedic setting. The sheer scale of the world and diversity of the areas, levels and enemies means you can get lost for hours exploring and experimenting discovering new secrets and new ways to play.