Warning: This article contains SPOILERS for The 100 season 6 finale.
The 100 season 6 finale ended with a shocking twist - but is Octavia really dead? After spending the last several weeks focused on the Primes and the fact that they use people with nightblood as hosts so that they can live forever, The 100 has disregarded the anomaly and its effect on Octavia - that is, until The 100 season 6 finale. Earlier in season 6, Octavia and Diyoza joined forces to bring down after they were essentially banished from Sanctum. It was their way of seeking redemption and forgiveness from everyone.
At one point in her journey, Octavia's hand was exposed to a temporal flare, which left her rapidly aging. So in order for her to survive, she went into the anomaly - but only after Diyoza had done so first, because she was chasing after her daughter, who in fact had yet to be born - and came out perfectly healthy. But that wasn't all; Octavia returned from the anomaly with her hair cleaner and even longer, which led Gabriel to believe that she was in there for a much longer period of time, not just a few seconds as it seemed to be for him.
In The 100 season 6 finale - after Clarke defeated the Primes and Madi regained control of her mind, along with Bellamy, Murphy, Echo, and everyone else on the ground took care of all the other people in Sanctum - we saw Gabriel, Bellamy, and Octavia return to Gabriel's hut. A device was hidden underneath the camp, which had been created by an unknown person thousands of years before, which featured symbols that were also tattooed on Octavia's back. As it turns out, everything is connected.
Even though Gabriel had been studying the device for 150 years, it wasn't until this moment that he knew the correct sequence of symbols to select in order to apparently trigger the anomaly to expand. Given that the symbols were on Octavia's back, it's most likely a sequence that allows people to enter and leave the anomaly freely; it could've been developed by the people who now exist within the anomaly itself.
After the anomaly expanded, it was then that an adult Hope appeared and stabbed Octavia. Right before Hope let go of Octavia, though, she apologized for failing to "get out of it," whatever that may mean. But what's even more surprising is that Octavia acknowledged her apology and responded, "Be brave. Tell him it's done." So it seems Octavia is at least somewhat aware of what's really going on with her and the anomaly.
It's unclear at the moment if Octavia is still alive, since her body seemed to disappear into the anomaly itself, but it's something that will certainly be addressed in the season 7 premiere episode. Furthermore, it doesn't seem as if Octavia's story is over, because she not only has a fundamental connection to Gabriel's device but also plays a part in The 100's new mystery regarding the anomaly as well as whoever took Diyoza in the first place.