How can I create a TRIGGER in MySQL from this Oracle syntax?
Check the MySQL documentation about the trigger syntax and examples.

You cannot have AFTER INSERT OR UPDATE triggers in MySQL. Define 2 triggers, one for INSERT and one for UPDATE. Another limitation is that AFTER triggers cannot change the updated rows, so you can use BEFORE triggers. Like this:

delimiter //
DROP TRIGGER IF EXISTS TRG_PRODUCT_REORDER_BEFORE_INSERT ;
CREATE TRIGGER TRG_PRODUCT_REORDER_BEFORE_INSERT
BEFORE INSERT ON PRODUCT
FOR EACH ROW
    BEGIN
        IF NEW.P_ONHAND &lt; NEW.P_MIN THEN
            SET NEW.P_REORDER = 1;
        END IF;
    END; //
delimiter ;


and similarly for the BEFORE UPDATE trigger