Why does the Torah use the word "Vaya'al" in connection with Yisrael's moving towards Og Melech ha'Bashan?
Rashi: Because whenever one travels northwards, one is 'ascending'.
Oznayim la'Torah: Because Og's land was mountainous. 1
See Oznayim la'Torah.
Why did Moshe not send a message of peace to Og, like he did to Sichon?
Ramban (on 2:34): Because Og attacked Yisrael before he had a chance to do so.
Da'as Zekenim, Hadar Zekenim and Rosh (on 2:26): Because, whereas most of Sichon's land was initially captured from Amon and Mo'av, Og's land was Eretz Refa'im (which was promised to Avraham), and Yisrael did not offer peace-terms to the seven nations. 1
