Below are mostly minor notes for which clarification is needed.

Conceptual: are we sure that ORACLE mode should invalidate native MariaDB constructs? We need some consistency and in my understanding following should still work in Oracle mode:
ELSE / ELSEIF
% operation (if possible)

2. MDEV-10343 sql_mode=ORACLE: Providing compatibility for basic SQL data types

following are valid build-in datatypes declaration in oracle which cause syntax error at the moment (needs to be documented or created another task):

these probably had to be implemented inside MDEV-10343

create or replace table t1(a binary_float);

create or replace table t1(a binary_double);

create or replace table t1(a long raw);

create or replace table t1(a NCLOB);

these need more effort or maybe just must be documented or separate task is needed

create or replace table t1(a varchar(30 byte));

create or replace table t1(a varchar(30 char));

create or replace table t1(a timestamp with local time zone);

create or replace table t1(a timestamp with time zone);

create or replace table t1(a interval day);

create or replace table t1(a interval year);

create or replace table t1(a BFILE);

create or replace table t1(a rowid);

create or replace table t1(a urowid(5));

this should give an error (oracle will return ORA-02017: integer value required )

MariaDB [test]> create or replace table t1(a number(2.2));

Query OK, 0 rows affected (0.24 sec)

3. MDEV-10411 Providing compatibility for basic PL/SQL constructs
3.6 Assignment operator
It doesn't sound intuitive that one may write "wait_timeout:=a", but "a:=wait_timeout" shows syntax error.

begin   declare a int;   begin     a:=wait_timeout;   end; end;;

3.7 Variable declarations
Oracle allows NOT NULL in variable declaration. Maybe we should allow that syntax as well?

begin   declare a NOT NULL int;   begin     a:=1; a:=NULL;   end; end;;

10. MDEV-10585 EXECUTE IMMEDIATE
In ORACLE the command supports 'RETURNING INTO' clause, we should probably document the difference?

sql_stmt := 'UPDATE emp SET sal = 2000 WHERE empno = :1

      RETURNING sal INTO :2';

EXECUTE IMMEDIATE sql_stmt USING emp_id RETURNING INTO salary;

12. MDEV-10588 TRUNCATE TABLE t1 DROP|REUSE STORAGE
Following clauses should be allowed in TRUNCATE command as well:
[ PRESERVE MATERIALIZED VIEW LOG | PURGE MATERIALIZED VIEW LOG ]

18. MDEV-10577 sql_mode=ORACLE: %TYPE in variable declarations
It is not clear what is expected to happen if table's column definition is altered after stored program creation.