How do I limit Oracle's memory use?
The problem was that when the database was installed, conflicting other parameters were set. So I needed to export those from the "spfile" to a "pfile", back them up, edit them, test them and import from the pfile to the spfile:

SQL&gt; create pfile='some/file/path' from spfile;

File created.


Then I set the conflicting parameters to zero:

...
--*.pga_aggregate_target=3270m
*.pga_aggregate_target=0m
...
--*.sga_target=9811m
*.sga_target=0m
...


I then try starting up again:

SQL&gt; startup pfile='some/file/path';

ORACLE instance started.
...


And check the memory_target and memory_max_target parameters as above. Once happy with things, to keep the settings I did:

SQL&gt; create spfile from pfile='some/file/path';

File created.