O'Brien, Oregon, nestled in Josephine County, experiences a Mediterranean climate with warm, dry summers and mild, wet winters. The average high temperature in July is around 90°F (32°C), while December brings an average low of 30°F (-1°C). The region is known for its annual rainfall of about 40 inches, with most precipitation occurring between November and April.

Hydrology constituents in O'Brien include the Illinois River, which flows nearby. Additionally, the region benefits from several streams and creeks that contribute to the local watershed. These water sources provide opportunities for fishing, swimming, and wildlife viewing.

Outdoor enthusiasts will find ample recreational opportunities in O'Brien. The surrounding area offers hiking and biking trails, perfect for exploring the beautiful landscapes and taking in the stunning views. Visitors can also enjoy camping, boating, and rafting on the nearby rivers. The diverse flora and fauna make O'Brien an excellent destination for nature lovers seeking an adventure in the great outdoors.