Native to the Pacific Northwest's woodlands, the Oregon grape holly (Mahonia aquifolium) is an open branching shrub, often with suckering shoots popping up from its roots. Slow-growing, it also is used in garden landscapes because of the attractive foliage, brightly colored flowers and of course the blue berries that look like grapes. This evergreen shrub grows best where it isn't exposed to dry soil conditions or drying and cold winter winds. It grow in U.S. Department of Agriculture hardiness zones 6 through 9, where winter low temperatures don't drop below -10 degrees Fahrenheit.
Examine the foliage of the plant in question. Oregon grape holly's leaves can be quickly identified. It has glossy green leaves that are 6 to 12 inches long, and comprise seven to 12 leaflets. Each leaflet is a tapering oval with spiny teeth on the edges (some say it looks like leaf of an English holly). In fall and winter the leaves often attain a maroon-purplish hue. The leaves are attached to the shrub's slender trunks in an alternating arrangement.
Look at the habit of the plant. Oregon grape holly's trunks/stems are upright with irregular side branching, creating a shrub that is typically between 3 and 6 feet in height. The width of the plant, including any bottom shoots from the roots, is around 5 feet. Keep in mind that young plants are smaller in size but generally have the same habit.
Search for any clues revealing the characteristics of the plant's flowers. The bright yellow flower clusters are dense masses and measure about 3 inches in size and appear on the tips of the upright shoots at the top of the plant. Oregon grape holly blooms in mid-spring (whereas many other species of Mahonia may bloom in summer, fall or midwinter).
Look for any remnants or development of fruits. After the yellow flowers wane, the tips of shoots form spherical berries (each up to 1/2 inch in diameter) in clusters. In late spring to midsummer these berries are green, but by late summer then begin to turn cobalt blue, and even deeper blue-black by winter.
Take a photo or a branch clipping with leaf or fruit to a plant nursery or botanical garden and ask for a horticulturist to assist you. These professionals have more extensive exposure to various plants and can be more specific in differentiating one species of Mahonia from another for absolute identification. They also can help identify the many cultivars (garden variations) of the Oregon grape holly that may be problematic to distinguish.