Depending on the severity of the theft charge and your past criminal history, if any, it is indeed possible to get both the arrest and the conviction expunged. In general, if you have no past criminal history, you can apply for an expungement 3 years after your date of conviction, but not sooner. So if you were convicted in 2009, you may have about another year or so left to wait.
Certain crimes cannot be expunged, but misdemeanor theft usually does not fall into that category.
Once you are eligible for expungement, a lawyer can likely handle that quickly and easily for you.
I see that you live in Marion County. Here is a link that may help you with your questions. The Process is basically the same in every Oregon County. The forms are basically the same as well. You do need to file the expungement in the county where the conviction occurred.
Now, while you can do the forms yourself. I usually advise that someone hire a lawyer to do this. If there are any snafu's you want to make sure that you have someone there to assist you. It is obviously important to make sure it's taken care of correctly, and most lawyers charge a pretty modest fee for an expungement, unless there are going to be obvious problems.