Would A state in the Oregon territory be allowed to join the US as a slave state?

Yes, a state in the Oregon Territory could potentially join the U.S. as a slave state, but it would depend on the political climate and the balance of power between free and slave states at the time of statehood. The Missouri Compromise of 1820 established a line of latitude (36°30′) that determined where slavery was permitted in the western territories, but the Oregon Territory was north of this line. Ultimately, the Oregon Territorial Legislature prohibited slavery in 1844, and Oregon entered the Union as a free state in 1859.