Yeah, it's kept democrats in control, amazing how that works. 
Your insinuation is flawed for several reasons, Chaos.
Prior to 2012, Oregon was the most consistently Republican west coast state. The Oregon Legislature approved vote by mail in local elections in 1981 and state elections in 1995, seventeen years prior to Democrats' control of the legislature.
As is the case in most of the U.S., voters in urban areas are more liberal, left-wing (democrats), and rural areas they are more conservative, right-wing (republicans). The bulk of Oregon's population is in its largest cities. this is why democrats have been in control of the Legislature and not because of mail in balloting.