David OReilly (b. 1985, Kilkenny, Ireland) is an artist based in Upstate New York whose work explores worlds and simulations: systems that unfold, behave, and resist conventional narrative. His practice spans animation, video games, and large-scale installation.

His short film Please Say Something (2009) won the Golden Bear for Best Short Film at the Berlin International Film Festival — the first computer-animated work to receive the prize. Its follow-up, The External World (2011), premiered at Venice and won over forty international awards. He has written for Adventure Time and South Park, and designed the fictional video games in Spike Jonze's Her.

In 2014 he released Mountain, a minimal ambient simulation. Everything (2017), a model of the universe in which every object is a playable character, won the Golden Nica at Prix Ars Electronica and became the first video game work to qualify for an Academy Award. Eye of the Dream (2018–2023), a 360° dome film and installation, was exhibited at the 21st Century Museum of Contemporary Art, Kanazawa. His most recent work, EACHOTHER (2026), is a live simulation of one million autonomous beings, with its first public preview at the 42nd Hamburg International Short Film Festival in June 2026.

His work has been shown at the Barbican Centre, NTT ICC, the Walker Art Center, IMMA Dublin, MAAT Lisbon, SFMOMA, and the Hong Kong Arts Centre, and has been the subject of retrospectives at MoMA New York, the Walt Disney Concert Hall, and New Chitose International Animation Festival. He is a member of the Academy of Motion Picture Arts and Sciences.