It's possible for a single object to contain multiple oscillator rotors.
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x=36, y=33
2o14b2o3b2o$obo13bo4bo4bo$19bobo3bobo$2bobo13b2obob2obo$3b2o17bo$5b2o$5bob
o2$7bobo$8b2o7$2o22bo$o22bobo$2b2o19bobo4b2o$21b3ob2o4bo$3bobo14bo10bob2o$21b
3ob2o3b2obobo$5bobo15bobo7bobo$30bobobo$7bobo20bo2bo$9bo16b2o3b2o$11bo13b
ob4o$10b2o13bo4bo$12b2o12b3o$12bobo13b2o2$14bobo$15b2o!
On the top line at the left is a 16-bit object which appears to be the smallest case, but here both rotors are period 2, so there's no new period. On the right is an 18-bit object with a Period 2 and a Period 3 rotor, making the object itself Period 6.
Historically, talking about a Pn oscillator has implied that an object has a single rotor, which is why in the second row, both objects may be Period 10, the one on the right (found by Dean Hickerson) is the "smallest oscillator" since it has a single rotor.
In some ways this mirrors the difference between "objects" and "pseudo-objects", in that the latter can be subdivided.
(My definition above could conflict with the "at least one cell must oscillate at the full period" one, if it's possible to have a composite rotor in which two rotors with different periods overlap and support each other. For example, a Period 6 which is actually a composite of Period 2 and Period 3. But I don't even know if that's possible, or if an example has already been discovered.)