I faced this exact same problem. None of the previous suggestions fixed the problem: I could enter 30m for duration, but it would be automatically converted to 1d as soon as I began editing a different task.
Turns out you also have to have “Granularity:” set to “Exact Scheduling” in order to allow scheduling dependent tasks shorter than a day all on a single day. This setting can be find in the Project Inspector settings (briefcase icon) under Project Info. The default was “Daily Scheduling”.
“Exact Scheduling” sort of makes sense now that I’ve turned it on, but it was not easy to figure out and the help wasn’t useful (especially since I couldn’t tell if OmniPlan was even able to do this).
I’m putting this note here in case any future user has a similar problem.