According to the Elder Scrolls wiki, if you miss an artifact the Oblivion Walker achievement can still be obtained by getting both The Ring of Hirscine and the Savior’s Hide from the Ill Met By Moonlight quest.
UESP has instructions for doing this:
It is entirely possible to obtain both the Ring of Hircine as well as
the Savior's Hide. In order to do so, first you must help Sinding
eliminate the remaining hunters. After doing so, speak with Sinding to
complete his quest objective, then kill him. When you remove his skin,
immediately avoid contact with the Hircine's spectral avatar (or back
out of the conversation) and exit the cave, leading you to the spirit
of the white stag, which will then reward you with the uncursed Ring
of Hircine. Now re-enter the cave and speak with Hircine's spectral
avatar to receive the Savior's Hide.