I have to wonder exactly where that quorum is specified.
Because US Senate Republicans already set the precedent for suspending quorum rules, at least in committee, when Democrats were stalling some of Trump's Cabinet nominees by voting to suspend quorum rules (one of the few things you can do without a quorum), and then advancing the nominees out of committee. Can't do that on the floor, though, I think.
From my understanding, the Oregon state constitution states what is required for a quorum, which requires two more members than all the Dems. The US Constitution states that a quorum is required for passing laws, but doesn't define how many Senators make up a quorum, so the Senate can define a quorum as whatever they want.
The US Constitution (Art. 1, Sec. 5, Clause 1) specifically requires that a quorum is a majority of members, for either house.
Oregon requires two-thirds, but also allows the legislature, without a quorum, to compel the attendance of absent members. Presumably it's under this power that the legislature requested that the Governor force the Republican legislators to return to work.
(And yes, state Democrats are two members short of a quorum in either house: 18-11 in the State Senate, with one vacancy after a Republican died in office, and 38-22 in the State House.)