OE3CEMA OE3 CEMA San Jose #WeAreOE3CEMA OE3 is the largest Construction Union in the United States. CEMA is a full affiliate of OE3. CEMA is the second largest union representing Santa Clara County employees. We represent over 2,000 middle managers, supervisors and other professionals. OE3 represents nearly 40,000 private and public sector workers.