Jamieson, Fausset and Brown on Obadiah 1:15

15. For--resumptive in connection with Ob 10, wherein Edom was threatened with cutting off for ever.

the day of the Lord--the day in which He will manifest Himself as the Righteous Punisher of the ungodly peoples (Joe 3:14). The "all" shows that the fulfilment is not exhausted in the punishment inflicted on the surrounding nations by the instrumentality of Nebuchadnezzar; but, as in Joe 3:14, and Zec 12:3, that the last judgment to come on the nations confederate against Jerusalem is referred to.

as thou hast done, it shall be done unto thee--the righteous principle of retribution in kind (Le 24:17; Mt 7:2; compare Jud 1:6, 7; 8:19; Es 7:10).

thy reward--the reward of thy deed (compare Is 3:9-11).