About O drive, please confirm that Web App can see the drive. Does the following work in Web Apps?
If, not the network driver is not mounted as O drive from Web Apps user (MwWebAppsGuestR2019a). In this case, hostname or local IP address of the network drive would work, for example, if the hostname is servershared and its IP address is 192.168.0.10, \\servershared or \\192.168.0.10 instead of O:\ would work.
About the file permission, please check the xlsx file can be accessed from Web Apps user (MwWebAppsGuestR2019a). You can check the file permissions from right click the xlsx file in File Explorer and go to Properties, then check the Security tab.