O'Donnell
O'Donnell, 38, signaled last month that he was interested in the seat currently held by Rep. Ellen Roberts, R-Durango. The seat will be open because Roberts is running for state Senate.
On the Republican side, businessman Lew Webb of Durango has entered the race, and Durango City Councilor Doug Lyon is "carefully considering" it.
O'Donnell is executive director of the National Conservation System Foundation. He was a delegate to the Democratic National Convention last year in Denver.
"I will work hard in the Legislature to ensure that the citizens of Southwest Colorado get their fair share of government services, I will fight any effort to take our water, and I will stand up for the needs of our rural communities," O'Donnell said in a statement announcing his candidacy.
The district encompasses Archuleta, La Plata, San Juan and eastern Montezuma counties, including most of the city of Cortez.