We are using Linux DB for our hosted solutions already since quite some time.
But i am afraid that the only certified platform for on-premise installations is still Microsoft Windows Servers.
the linked document was updated January 2018, and i think its still the most current one.
As of this writing OPERA versions lower than 5.0.04.03 are only certified on Microsoft Windows 2008r2.
Microsoft Windows Server 2012r2 is only certified for OPERA Versions 5.0.04.03 and above including Database, Application, and Single Servers.
Further to consider for your version is following advisory :
You must use an 11g version (for example, 188.8.131.52 or 184.108.40.206) of the Oracle database as the underlying database for Oracle Hospitality OPERA Property Management Version 220.127.116.11.
as said, Oracle 11g on Linux works perfectly fine in our DC from a technical side. but i did not see any documents that this is certified for on-premise customers as of today.