I have a couple of questions.... Firstly, do the bad guys reveal themselves before the end of the game and talk to the assassin or does he simply rely on himself. Also, does Oberon reveal himself at the end of the game too? Lastly, can the good guys talk when the assassin is choosing, or must it remain completely silent?
At the end of the game no role cards are reveled until the Assassin declares who he thinks Merlin is. The Assassin should be able to remember who the evil players are and determine who Oberon is based on game play.
Without revealing any Character cards, the Evil players discuss and the player with the Assassin character card will name one Good player as Merlin.
There is nothing mentioned about the good players remaining silent or being able to speak.
Although it may not directly advise doing so in the rules. I would strongly recommend that the assassin's character card only is revealed at the start of the end game. We have played a number of games and found that without doing this the good players (particularly those new to the game) might take an accusation from another evil player as being the Assassin's choice, which can ruin that game if merlin reveals themself before the assassin disagrees.
I would always suggest, particularly as it has no bearing on the rest of the game, that the Assassin is revealed. After the final mission, whoever has read out the instructions during the rest of the game (or whoever is generally "running" the game as it were) has a similar instructional statement that the Assassin reveals their identity and discussion and naming of Merlin begins.
I've always played that the evil team can discuss all they want, but ultimately the decision is up to assassin. I didn't realize it was a rule not to reveal character cards so we usually had the entire evil team reveal themselves and actually use that to try and logically figure out who Merlin was.
For example if someone wasn't suspicious of the person who was Mordred till the very end, but immediately was accusing the other evil people this would obviously be valuable information you wouldn't necessarily know about unless Mordred also revealed who they were.
Usually the good players sit silently while the entire evil team discusses and so far this has worked really well for us. Sometimes discussion is helpful and sometimes it steers the assassin away from the right choice, but I find it just makes things more interesting.
And as soon as the assassin is reveal the rule is they must say "I choose to assassinate ________" to make their final decision so there's no confusion.