OHV tags are required to ride OHV areas, regardless of vehicle.
OHV tags are *NOT* required for riding private land, forest service roads, etc.
Oregon OHV areas:
Areas 13, 14, 15, 16, and 36 are closest, but ot necessarily close. Some rides may link through one or more.
Info on OHV tags:
You must display an ATV permit if you are operating your ATV on public land (and even then, the land must be specifically designated for ATV use). The ATV Permit Fee is $10.00 (Most permit agents charge an extra .50), and it is valid for two years. See the latest permit agent list. There are three classes of ATVs (see below).
Note ATV and OHV are to be used interchangably. Public land "designated for ATV use *IS* an OHV area.
They correct this in the reciprocity section:
An ATV/OHV operating permit that is issued in another state shall be honored in the State of Oregon if the issusing state also honors an Oregon ATV operating permit. The ATV must have a resident state ATV/OHV operating permit or a State of Oregon ATV operating permit to operate the ATV on designated ATV areas. A State of Oregon ATV operating permit may be issued for all terrain vehicles owned by a resident of another state that does not have a permitting program. For a list of states that have reciprocity with Oregon, please visit the Reciprocity Information Page.
OHV permits are available at every moto dealership and powersport store I have looked for one in.
EDIT: LOCATIONS LINK