No, ealier mission has to be done, it was confirmed by Cipsoft here
The following quests of the Grimvale questline must be completed to be able to start the new quest: “The Curse Of The Full Moon”, “The Curse Spreads” and “An Ancient Feud”.
So it means you won't be able to start new missions (and access to oskayaat) without done earlier missions
Additional information confirmed by Cipsoft that Feroxa doesn't need to be killed to access Oskayaat
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