Yeah. The OC starts at level 1 and takes the character to the late teens. SoU starts at level 1 and takes the character to the low-mid teens and HotU assumes that same SoU character is being played. (Some people play that character through one of the
SoU-HotU "bridge modules" to get him to ~15th level to start HotU.) The OC henchmen in HotU (and the whole storyline) know about the OC, but they all assume that the character played is the one from SoU.
FWIW, HotU will play okay with the OC character, as long as you don't mind that some of the dialogue refers to you as the SoU character who (with Drogran) saved the region from the threat of a risen Undrentide and no one knows you are the hero of Neverwinter that they sometimes talk about. Also, a character taken through the OC is likely to finish at a higher level than is intended to play in HotU, so it might not be very challenging for that character (and HotU isn't really that hard anyway).
BTW, just as a personal preference, I prefer taking the SoU character through HotU because some of the magic items one can find in SoU are more interesting, especially for arcane casters.