It looks like the relevant wording is here:
From what I read, it doesn't say that "only" belt holsters are allowed. Just that a belt-holstered weapon shall not be considered "concealed". I believe this is to reflect an idea somebody brought up in another thread this morning - that since a proper holster covers a good portion of the gun, and many jurisdictions don't allow concealment of any portion of the gun, this clause simple excludes the holster as "concealment". That way, you can openly carry with an obvious holster, but not be prosecuted just because it partially covers the gun.166.250. (1) Except as otherwise provided in this section or ORS 166.260, 166.270, 166.274, 166.291, 166.292 or 166.410 to 166.470, a person commits the crime of unlawful possession of a firearm if the person knowingly:
(3) Firearms carried openly in belt holsters are not concealed within the meaning of this section.
It seems excessive wording to me, but you know there had to be some reason to include it that way. After all, they had to put a warning on clothing irons to NOT iron clothes while wearing them.
EDIT: Ugh...now that I think about it, he may be right. The wording specifically says "belt" holsters. You know - I think a 2A-friendly lawyer in Oregon is your best bet to find the correct interpretation.