zag, it's not entirely true that "state land is off-limits". The commonly referred to chart, that people get this information from, is usually one of these type circulating lists:
Federation of Metal Detector & Archaeological Clubs Inc.
Bear in mind that for state PARKS only. Not all "state
land" (because not all state land is "state
park" land). As you can see they're not all "off-limits". And also note that this has no bearing on city or county or private or federal lands. Take your state for instance (Oregon). You can see that it's ok, with a "permit". Not sure what's involved in getting that (probably futile to try, as maybe they'll say you have to be an archaeologist, perhaps). But look at CA on that list, for example, notice it says "yes with permission from park office". (very similar sounding to your OR info there, eh?) Yet I can tell you for a fact, that some state parks were, and are, detected here, and no one cares. For example: state of CA beaches (which are administered by the very same parks dept afterall) are routinely detected here, and no one cares. And no, no one goes asking permission at the kiosk, etc...
You have to keep in mind, how such lists (that led to the scary stories of which you are now worried about) came into being, TO BEGIN WITH: Someone(s), decades ago, went and asked. For example one such compendium list, was a book by Doc R. Grim called "Treasure Laws of the United States", which came out in the early 1990s. The idea of the book, was so that users could look up any state, in alphabetic order, to see the applicable laws there. And thus people who travel in RV's or whatever, could carry that book, and show busy-body gripers, in case they were approached. Or conversely, to avoid other states, and so forth. The way the author got the info, was he xeroxed 50 form letters, and sent them out to each state capitol head office of parks dept. The letter said something to the effect of: "what are your laws regarding the use of metal detectors in your state parks?" Then when he got back all 50 replies, he made them into this nifty book. Sounds logical enough, right? I mean .... who better to ask, than the states themselves!

But an interesting thing happened, when lists like this started making the rounds: There were some dire sounding states (with outright no's, or spelling severe limits). Yet they had, up-till-then, just been detected, and no one had ever cared.

Barring obvious historic monuments, of course, and assuming you weren't being a nuisance in some other way. But now here's a list and a letter from the head-of-parks saying "no"? Well gee, that was "news" to all the old-timers, eh?
Here's what had occured: Put yourself in the shoes of whomever answers a letter, such as that, back in the 1980s or '90s. What do you think the "safe" answer is going to be? Perhaps the person fielding such an inquiry had never even given the matter thought before! So what will he do? He'll look too and fro through all their text and verbage, to see what "applies to this pressing question". And thus, presto, various verbage about cultural heritage, alteration and defacement, lost & found, harvesting and collecting, and all sorts of wonderful things were found to give no's. Because, think of it: there are .... GRANTED, some parks, in any given state, that are admittedly sensitive historical monuments, right? So the person answering simply can't go into detail, like "yes at these 30, but no at those 4". Or "yes on the beach and grass at the lake, but avoid the historic cabin on the north shore", etc... Obviously the much easier answer, to the "pressing question" was, "no", or "inquire at each kiosk". But as I say, we're talking about parks that had, up till then, probably just been routinely detected, and no one had ever cared! (unless, as I say, there were something unique to a particular place). Thus it was a clear case of "No one cared,
till you asked" routine

And I can tell you for a fact, that (just like the example I gave of CA) that, despite such lists (and scary mantras like Casca cites) detecting is still going on.
As far as Casca's saying that some city's may require a permit: This is very very rare. Very few cities, across the united states, have ever implemented such a thing. In my entire state of CA, for instance, I can think of perhaps 3 (and two, yes "all two of them" counties), out of many hundreds of cities and out of 58 counties. But if you still worry that your city or county might have something, here's what you do: Do
NOT go asking desk-bound bureaucrats "can I?". Lest you merely get more of the "no one cared till you asked" routine. Instead: You look it up for yourself. City and county muni codes, rules, laws, etc... are all public info, available for public viewing SOMEWHERE. Either at city hall on the front desk in binder form, or perhaps on the city or county website, etc.... If you see nothing there that says "no metal detecting", then PRESTO, it therefore must not be prohibited.