Expunctions remove distinct criminal items from your arrest/criminal record completely and applies to dismissals, acquittals and an arrest without prosectution. (According to your state law, there may be a statutory waiting period for requesting expunction if you are eligible). Sealing your record only protects it from being disclosed to certain individuals and privtate entitiies. Thus, if you have your recored sealed, it will not be sealed from law enforcement officicials and goverment agencies on local, state or federal levels.
I agree with my colleague from Houston regarding the distinction between sealing and expunging a record. However, in Oregon, you can also set-aside ("expunge") convictions as long as you meet certain statutory requirements. The guide I link later in this answer walks you through these requirements for expunging both arrest records and convictions. There's not really a process for sealing adult records just because you request it, so your option is probably going to be expungement. If this is granted, in the eyes of the law, you never were arrested or convicted in the first place.
The process is quite different for juvenile and adult records. You should consult with an attorney who handles expungements to see if you qualify and what options you have. I'm attaching a guide I wrote on the subject (this is only for adult convictions/arrests): http://www.avvo.com/legal-guides/ugc/expungement-101-five-easy-steps-to-see-if-you-qualify
If you would like specific answers regarding your situation, feel free to give me a call on Monday.
My responses to posts on AVVO are not legal advice, nor do they create an attorney-client relationship. In order to provide true (and reliable) legal advice, an attorney must be able to ask questions of the person seeking legal advice and to thus gather the appropriate information. In order for an attorney-client relationship to exist, you and I both have to agree the the terms of such an agreement.
Expungement is the process by which a criminal conviction and/or arrest is destroyed and erased from court records. The Oregon Statute regarding this process is ORS 137.225. There is a slightly different process when dealing with (1) an arrest but no formal charges were filed; (2) an arrest, formal charges were filed but later dismissed; and (3) an arrest, formal charges filed resulting in a conviction. Some counties have an Expungement Clerk at the courthouse. You can also find information on county circuit court websites. You may want to consult with an attorney before beginning the process to ensure that you are eligible.