Oath - A reference to the oath of allegiance taken to Solomon at his accession to the throne (the margin of 1-Chronicles 29:24).

To keep the king's commandment - This sentence would be better translated, I keep the mouth of the king; I take good heed not to meddle with state secrets; and if I know, to hide them. Or, I am obedient to the commands of the laws; I feel myself bound by whatever the king has decreed.
In regard of the oath of God - You have sworn obedience to him; keep your oath, for the engagement was made in the presence of God. It appears that the Jewish princes and chiefs took an oath of fidelity to their kings. This appears to have been done to David, 2-Samuel 5:1-3; to Joash, 2-Kings 11:17; and to Solomon, 1-Chronicles 29:24.

I [counsel thee] to keep the king's (c) commandment, and [that] in regard of the oath of God.
(c) That is, that you obey the king and keep the oath that you have made for the same cause.

I counsel thee to keep the king's commandment,.... Or, "to observe the mouth of the king" (w); what he says, and do according to it when it is agreeably to the law of God, and according to the laws of the kingdom, by which he is to govern; for kings are to be honoured, obeyed, and submitted to, in the lawful discharge of their office: and such counsel and advice as this is wholesome; and, being taken, contributes much, as to the honour of kings, so to the good of kingdoms and states, and to a man's own peace and comfort. Aben Ezra supplies it,
"I command thee, or I admonish thee;''
for it may be either a charge, or art advice, respecting this and what follows. Jarchi supplies and paraphrases it thus,
"I have need, and am prepared, to observe the mouth (or keep the commandment) of the King of the world;''
and so Alshech,
"observe that which goes out of the mouth of the King of the world.''
And indeed, to understand it, not of an earthly king, but of the King of kings, as it is understood by other interpreters also, suits better with what is said of this King in the following verses; whose commandments, which are not grievous, but to be loved above fine gold, should be kept from a principle of love, without mercenary and selfish views, as they are delivered out by him, and to his glory; and such a charge as this should be attended to, and such counsel be received;
and that in regard of the oath of God; who has swore, that if his children forsake his law, and walk not in his statutes, he will visit their transgressions with a rod, and their iniquities with stripes; and therefore should be careful to keep his commandments, Psalm 89:30. Those who interpret this of an earthly king, by the oath of God understand the oath of allegiance and fidelity to him, taken in the name and presence of God, and therefore for conscience's sake should obey him: or render it, "but so that thou observest the manner of the oath of God" (x); or takest care to obey him; or do nothing in obedience to kings, which is contrary to the will of God; for God is to be obeyed rather than men, Acts 4:19; especially, and above all things, that is to be regarded.
(w) "os regis observes", Tigurine version, Pagninus, Mercerus; "observa", Montanus, Vatablus, Junius & Tremellius, Piscator, Cocceius, Gejerus, Rambachius. (x) "sed, ita quod ad Deum attinent, observes rationem juramenti Dei", Varenius; "attamen, supra serve verbum juramenti Dei", Gussetius, p. 605.

the king's--Jehovah, peculiarly the king of Israel in the theocracy; Ecclesiastes 8:3-4, prove it is not the earthly king who is meant.
the oath of God--the covenant which God made with Abraham and renewed with David; Solomon remembered Psalm 89:35, "I have sworn," &c. (Psalm 89:36), and the penalties if David's children should forsake it (Psalm 89:30-32); inflicted on Solomon himself; yet God not "utterly" forsaking him (Psalm 89:33-34).

The faithfulness of subjects, Koheleth says, is a religious duty: "I say: Observe well the kings' command, and that because of the oath of God." The author cannot have written Ecclesiastes 8:2 as it here stands; אני hovers in the air. Hitzig reads, with Jerome, שׁמר, and hears in Ecclesiastes 8:2-4 a servile person speaking who veils himself in the cloak of religion; in Ecclesiastes 8:5-8 follows the censura of this corrupt theory. but we have already remarked that Ecclesiastes 8:2 accords with Romans 13:5, and is thus not a corrupt theory; besides, this distribution of the expressions of the Book of Koheleth between different speakers is throughout an expedient resting on a delusion. Luther translates: I keep the word of the king, and thus reads אשׁרּ; as also does the Jeremiah. Sanhedrin 21b, and Koheleth rabba, under this passage: I observe the command of the king, of the queen. In any case, it is not God who is meant here by "the king;" the words: "and that because of the oath of God," render this impossible, although Hengst. regards it as possible; for (1) "the oath of God" he understands, against all usage, of the oath which is taken to God; and (2) he maintains that in the O.T. scarcely any passage is to be found where obedience to a heathen master is set forth as a religious duty. But the prophets show themselves as morally great men, without a stain, just in this, that they decidedly condemn and unhesitatingly chastise any breach of faith committed against the Assyrian or Chaldean oppressor, e.g., Isaiah 28:15; Isaiah 30:1; Ezekiel 17:15; cf. Jeremiah 27:12. However, although we understand mělěk not of the heavenly, but of an earthly king, yet אשׁמר does not recommend itself, for Koheleth records his experience, and derives therefrom warnings and admonitions; but he never in this manner presents himself as an example of virtue. The paraenetic imper. שׁמר is thus not to be touched. Can we then use ani elliptically, as equivalent to "I say as follows"? Passages such as Jeremiah 20:10 (Elst.), where לאמר is omitted, are not at all the same. Also Ezekiel 34:11, where הנני is strengthened by ani, and the expression is not elliptical, is not in point here. And Isaiah 5:9 also does not apply to the case of the supposed ellipsis here. In an ingenious bold manner the Midrash helps itself in Leviticus 18 and Numbers 14, for with reference to the self-introduction of royal words like פרעה אני it explains: "Observe the I from the mouth of the king." This explanation is worthy of mention, but it has little need of refutation; it is also contrary to the accentuation, which gives Pashta to ani, as to ראה, Ecclesiastes 7:27, and לבד, Ecclesiastes 7:29, and thus places it by itself. Now, since this elliptical I, after which we would place a colon, is insufferably harsh, and since also it does not recommend itself to omit it, as is done by the lxx, the Targ., and Syr., - for the words must then have a different order, המלך פי שׁמר, - it is most advisable to supply אמרתּי, and to write אם אני or אני אם, after Ecclesiastes 2:1; Ecclesiastes 3:17-18. We find ourselves here, besides, within an I section, consisting of sentences interwoven in a Mashal form. The admonition is solemnly introduced, since Koheleth, himself a king, and a wise man in addition, gives it the support of the authority of his person, in which it is to be observed that the religious motive introduced by ו explic. (vid., Ewald, 340b) is not merely an appendix, but the very point of the admonition. Kleinert, incorrectly: "Direct thyself according to the mouth of the king, and that, too, as according to an oath of God." Were this the meaning, then we might certainly wish that it were a servile Alexandrian court-Jew who said it. But why should that be the meaning? The meaning "wegen" because of, which is usually attributed to the word-connection עלדברת here and at Ecclesiastes 3:18; Ecclesiastes 7:14, Kleinert maintains to be an arbitrary invention. But it alone fits these three passages, and why an arbitrary invention? If על־דּבר, Psalm 45:5; Psalm 79:9, etc., means "von wegen" on account of, then also על־דברת will signify "propter rationem, naturam," as well as (Psalm 110:4) ad rationem. שׁב אל is, as elsewhere שׁב יה, e.g., Exodus 22:10, a promise given under an appeal to God, a declaration or promise strengthened by an oath. Here it is the oath of obedience which is meant, which the covenant between a king and his people includes, though it is not expressly entered into by individuals. The king is designated neither as belonging to the nation, nor as a foreigner; that which is said is valid also in the case of the latter. Daniel, Nehemiah, Mordecai, etc., acted in conformity with the words of Koheleth, and the oath of vassalage which the kings of Israel and Judah swore to the kings of Assyria and of Babylon is regarded by the prophets of both kingdoms as binding on king and people.

The oath - Because of that oath which thou hast taken to keep all God's laws, whereof this of obedience to superiors is one.