Map showing the locations of known or suspected glaciers in Oregon, which are indicated by the white dots. The three main clusters are on Mt. Hood, Mt. Jefferson and the Three Sisters region. 

According to U.S. Geological Survey maps, Oregon has around 40 glaciers. However, these maps are rarely updated and largely rely on photographic surveys from the 1950's. Depending on when a photograph was taken, it can be difficult to distinguish between a snowfield and a glacier. Consequently, it is unknown how many glaciers exist in Oregon. Likewise, it is unknown how many glaciers existed in Oregon 100 years ago, prior to major human carbon emissions that have caused global warming. 

Based on the U.S.G.S. mapping, glaciers are found in the following mountain regions: 

Mt. Hood

Mt. Jefferson

North Sister

Middle Sister

South Sister

Broken Top

Wallowa Mountains

Mt. Thielsen

However, there is evidence that in the recent past that glaciers also existed on

Three Fingered Jack

Mt. Bachelor

Diamond Peak

Mt. McLoughlin

During ice ages, the last of which reached its peak 20,000 year ago, Oregon glaciers coalesced into an ice cap that covered much of the Cascades. However, the actual dimensions of this ice cap are poorly to not constrained. Similarly, glaciers in the Wallowa Mountains expanded, filling all the valleys of the range. Glaciers also grew in the Strawberry and Blue Mountains as well as in the Steens Mountains. Basically, Oregon used to be a much more icy place. 

Map showing the ice age ice cap that covered the Oregon Cascades and Mt. Hood. Note the course nature of the margins; this is not real and is rather due to the lack of actual data to constrain the extent of the ice cap. Figure from Mountain (1978, U. Oregon M.S. Thesis)