Oswaldo asks Chris Mannix if he's the one with the "Lincoln Letter". How could Oswaldo have known that someone in the party had that letter before S.L. Jackson (Warren) entered Minnie's, considering Warren was never expected to be there.
Is there some explanation for this or is this just a massive plot hole??
I wondered about that, too. The only thing I came up with was that maybe it was well known about the letter? John Ruth asked Warren in the stagecoach if he "still had it", which means he has known about the letter for some years. So I was thinking that Daisy's gang could have heard about it through the grapevine, so to speak. But it isn't known at first that Oswaldo was part of her gang, so it doesn't quite make sense, I agree. Also, you're right about Warren not having supposed to have been with them, so that doesn't make sense.
I was under the impression that John Ruth had told them about it when he walked in. Remember, he asked Warren and Mannix to both help with putting the horses in the stable. It's implied that Ruth had told them that SOMEBODY else was in the party and that one of them had a letter from Lincoln.
Remember this exchange:
Oswaldo: "Are you the one with the Lincoln Letter"
Ruth: "No, not him, the N%"
Oswaldo: (not exact): "The N% has a letter from Lincoln"
Mannix: (not exact): "The N%...has a letter...from Abraham Lincoln?"
Remember, the crew was waiting to ambush Ruth at the right time, and they did not expect any more than 3 people (Daisy, Ruth, and O.B.). They probably (smartly) inquired if there was anyone else once Ruth and Daisy entered the Haberdashery, while O.B., Warren, Mannix, and Bob were tending to the horses and the line to the outhouse. Ruth probably explained to them the general details about the size/composition of his party. This would have been offscreen, of course.