See also this older thread (and linked pages) Standardizing Units for Measurement - #11 by Vojtech_Huser
If a given measurement (e.g. weight) is mixture of 2 units (lb and kg) - since 2017 the community suggested a solution to have it ideal for analysis. (at ETL time harmonize to a single unit).
AND have all users of OMOP target THE SAME target unit such that multi site analyses work well.
AMIA 2018 poster https://www.researchgate.net/publication/378737424_Real_World_Database_for_Validation_of_Units_for_Clinical_Laboratory_Tests
2017 poster https://www.ohdsi.org/web/wiki/lib/exe/fetch.php?media=resources:huser-2017-ohdsi-symp-units.pdf