Trailforks is a great website that includes trails for mountain biking and other activities. It is a rapidly growing database of 149,905 trails in over 101 countries.
Local mountain biking options:
Pendleton Adventure Trails Recreational Area (PATRA)
Mount Emily Recreational Area (MERA)
Echo West Vineyard
Umatilla County road cycling opportunities:
Pendleton on Wheels map of rides
Blue Mountain Scenic Bikeway (century loop)
Northeastern Oregon (including road, gravel, trail riding)
Oregon Trail History
Blue Mountain Interpretive Park aka Oregon Trail Interpretive Trailhead
Experience the Oregon Trail as the pioneers did!