Really? I though RLBL were as slim as BoO? I have no experience with BoO, but I got the sense that I'd have to size up to L in them (and they'd still likely be too short). I wear my normal neck size in RLBL and they are good about giving you enough chest room but providing enough waist suppression to fit slim. I really like that their pleats are at the outsides of the shoulders as opposed to center pleats. Having them at the edges allows more shoulder room.