"and Og the king of Bashan, who dwelt in Ashteroth": (Even) if Og were not formidable, but he lived in Ashteroth, it would be difficult (to defeat them), the (inhabitants of the) land being formidable. And (even) if the land were not formidable, but Og dwelt therein, it would be difficult, the king being formidable. How much more so when both the king and the land are formidable!
"Ashteroth": hard as horns (karnayim), (its full name being Ashteroth Karnayim.) "in Edrei": the site of the war.
(Devarim 1:5) "Across the Jordan, in the land of Moav, Moses ho'il (began) to explain": "hoalah" is "beginning," as in (Judges 19:6) "Begin ("hoel") now and stay overnight so that you will be refreshed," and (I Chronicles 17:27) "and now you have begun ("hoalta") to bless the house of your servant to be before You forever." And the sages say: "hoalah" is "swearing," as in (Shemoth 2:21) "And Moses swore ("vayoel") to remain with the man," and (I Samuel 14:24) "And Saul beswore ("vayoel") the people, saying 'Cursed be the man who will eat bread until the evening when I will be avenged of my enemies.'"