In hindsight, I guess I shouldn't have used the "Enterprise GIS' category for my question. It's misleading, but it's the most relevant category that I came across.
I don't actually want to make an enterprise geodatabase. I just want to have an spatial Oracle database, that has
some sort of ESRI component, so I can develop my skills in an Oracle/ESRI environment at home. It would be nice to have either geodatabase functionality or st_geometry (or both). If st_geometry isn't possible, I could live with using SDO_GEOMETRY/Oracle Locator instead.
In summation, the database must be Oracle, and needs to have an ESRI component. Other than that, my requirements are flexible.
Forgive my ignorance, but Oracle database requirements for ArcGIS 10.3.x—Help | ArcGIS Desktop doesn't explicitly say that Oracle Express is -- or isn't -- supported, does it? Is it possible that Oracle Express isn't officially certified by ESRI for geodatabases, but can work, under certain circumstances?
The reason I ask is I found evidence of Oracle Express geodatabases here:
FAQ: Can ArcGIS Server 9.3 Enterprise (all editions) be used with Oracle Express Edition (XE)?
Oracle Express 11g support