Suppose a group of drivers want to block all regular use of a particular roundabout, so they all just start driving in it in circles. Once they're all in, the roundabout is useless, because there are no car-length gaps between them. Are they doing anything illegal? None of them are individually obstructing traffic, and traffic already in a roundabout has right-of-way over traffic trying to pull in.
They are impeding traffic, a Class D traffic violation.
Oregon Traffic Code 811.130:
Impeding traffic; penalty. (1) A person commits the offense of impeding traffic if the person drives a motor vehicle or a combination of motor vehicles in a manner that impedes or blocks the normal and reasonable movement of traffic.
A Class D violation is punishable by a fine of $65-$250, with the presumptive amount being $115.
A more serious potential charge would be disorderly conduct in the second degree (ORS 166.025), a Class B misdemeanor. This would apply if "with intent to cause public inconvenience, annoyance or alarm, or recklessly creating a risk thereof, the person [...] obstructs vehicular or pedestrian traffic on a public way;"
As far as "none of them are individually obstructing traffic", I might disagree. If the roundabout has plenty of space for other cars, then remaining in the roundabout unnecessarily might not in and of itself impede traffic. But here the roundabout is entirely filled with cars (those of the other drivers participating in the scheme). When there is no other available space in the roundabout, then I would argue that by remaining in it, you are impeding traffic: if you personally were to exit, then one more car could enter, which they otherwise cannot.
There is also a principle of joint criminal liability. Assuming that these drivers are participating in the blockade by common agreement, they have criminal liability for each other's conduct under ORS 161.155. Then under ORS 161.150, each one is guilty of a crime if it is "committed by the person’s own conduct or by the conduct of another for which the person is criminally liable, or both." Here it is their collective conduct that commits the crime of impeding traffic or disorderly conduct, and so they are all guilty.
137: Penalty for wilful obstruction.
If a person, without lawful authority or excuse, in any way wilfully obstructs the free passage along a highway he is guilty of an offence and liable to a fine not exceeding [£1000]
Note that there is nothing in the act that states that an obstruction must be immobile.
The question was about Oregon but to give a perspective from other places-
In Sweden the law is
'En förare får inte utan giltigt skäl köra med överdrivet låg hastighet,plötsligt bromsa eller på något annat sätt hindra andra förares körning.'
'A driver must not, without a valid reason, drive at excessively low speeds, suddenly brake or in any other way obstruct other drivers' driving.'
And in Israel
כל עובר דרך חייב להתנהג באופן שלא – (1) יקפח זכותו של אדם להשתמש שימוש מלא באותה דרך; (3) יפריע את התנועה ולא יעכבנה;
Every passerby must behave in a way that will not - (1) deprive a person of the right to make full use of the same way; (3) interfere with traffic and will not delay it;