I can't confidently speak to anything outside of a Norse context, but luckily that's the context of the Oseberg burial!

We don't have a lot of accounts of human sacrifice in any detail - the sagas and other historical evidence is fairly sparse on the topic (Ragnarssona þáttr describes a fictional "Blood-eagle" sacrifice, but that's specifically vengeance, and one account in Landnamabok) and the mythological sources are very sparse - Havamal claims that Odin sacrificed "his self to himself" via hanging and stabbing, but that doesn't tell us anything about human experiences.

There aren't a lot of foreign sources that go into detail. Adam of Bremen says that humans and animals were hanged on the forests at Gamla Uppsala, and were allowed to hang for the duration of the annual gathering, and Al-Ghazali claims that infanticide was practiced at Haithabu (Hedeby), but neither of them describe the rituals in any detail. That pretty much just leaves us with Ibn Fadlan for our textual sources that describe it in any reliable detail. Luckily, Ibn Fadlan answers your question pretty perfectly! In his voyage up the Volga, while staying with a Rus' community, he witnessed a funeral of a high-ranking noble and the funeral rites, including the ritual sexual assault and sacrifice of a female enslaved person. In it, he claims that they asked for a volunteer for the sacrifice, but once someone did volunteer, she could not retract that even if she wanted to. She was then lifted 3 times over an archway to see into the spirit world and find the spirit of the deceased and his kin, was repeatedly assaulted by the men of the community, and then was sacrificed and placed in the ship with the noble's corpse along with dog and horse sacrifices.

The clause that they couldn't back out is, of course, revealing, and indicates that yes, the victim was aware and/or realized that they were going to die. In fact, Ibn Fadlan claims that

So when this man died they said to his slave women: ‘Which of you wants to die with him?’  One of them answered, ‘I do.’ From that moment she was put in the constant care of two other women servants who took care of her to the extent of washing her feet with their own hands.  They began to get things ready for the dead man, to cut his costume and so on, while every day the doomed woman drank and sang as though in anticipation of a joyous event.

This does not, of course, mean that this necessarily means that the sacrifice is totally consensual - part of the ritual is drugging the victim and making noise.

They then led her to the ship but did not allow her inside the tent.  Then a number of men carrying wooden shields and sticks arrived, and gave her a beaker with nabid.  She sang over it and emptied it.  The interpreter then said to me, ‘Now with that she is bidding farewell to all her women friends.’  Then she was given another beaker.  She took it and sang a lengthy song; but the old woman told her to hurry and drink up and enter the tent where her master was.  When I looked at her she seemed completely bewildered.  She wanted to enter the tent and she put her head between it and the ship.  There the woman took her head and managed to get it inside the tent, and the woman herself followed.  Then the men began to beat the shields with the wooden sticks, to deaden her shouts so that the other girls would not become afraid and shrink from dying with their masters.

We also don't know that this particular sacrifice ritual is applicable across a Norse context - this is a cremation burial, like the mythological burial of Baldr, whereas something like the Oseberg burial is internment, implying some sort of cosmological shift. However, it's likely that victims of human sacrifice knew that they would die well before it actually happened - it's just.. kinda hard to hide a massively public funerary ritual where animals are also being slaughtered. Sadly, the emotions they must have felt, both negative and potentially positive, are lost to us.

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As a final side note about the Oseberg burial, while "servant + master" is the most common interpretation, it's actually very much uncertain who was who - there is one old female skeleton and one significantly younger female skeleton, but they both had a fairly high-status life, though the elder one suffered from cancer and a childhood injury which she survived, and neither skeleton shows signs of trauma at time of death. So, it's definitely possible that neither of them were servants, and it's instead a high-status sacrifice or a double-burial!