On Mac, you can enter an accented "o" by holding down the "o" key and then selecting the appropriate variant:
RackMultipart20190422104201wc8-5523673d-3bbb-4cc3-a8bf-34c3b430a803-1468696361.png
But when you do that in the Metadata panel's Caption field, you just get a plain "o".
However, you can use the Mac's Input menu to enter an accented "o" (both the Latin Small Letter O With Acute and the Latin Small Letter O followed by Combining Acute Accent).
Screen recording:
https://www.dropbox.com/s/zkyp4pfw8031vgt/holding-down-o-key.2019.04.22.mov?dl=0
Tested on LR 8.2.1 / Mac OS 10.14.3.