Oracle compatibility can only be set at the time of provision. It can't be changed afterwards. If you need to change the setting, you would need to raise a Ticket and see if support will copy your data into a new instance. Alternatively, provision a new instance yourself, checking the Oracle compatibility option, and copy your data over.
A quick way to check if you have compatibility mode enabled is to look at the following parameters
select distinct name, value from sysibmadm.dbcfg where name like '%compat'
If they are all on, then your database was created in Oracle Compatibility mode
NAME VALUE
--------------- -----
date_compat ON
number_compat ON
varchar2_compat ON
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