The Hebrew Bible has no word for "conscience." Instead, the word "heart" is used in ways that resemble our understanding of "conscience."
In places, the Old Testament envisions the heart as a moral guide. David prays for Solomon to have "a perfect heart, to keep thy commandments" (1 Chron. 29:19) and Solomon prays for "an understanding heart to judge Your people to discern between good and evil." (1 Kings 3:9) Jeremiah sees a day when God will make a "new covenant" with his people and will put his "law within them and on their heart." (Jere. 31:31-34).
The strongest impulses of the heart, however, are predominantly viewed as untrustworthy and deceitful (Jere. 17:9). Rabbinic Judaism distinguished two drives within the human heart. An evil inclination (Yetzer Hara) rooted in essential biological drives that issue in a psychological proclivity toward selfishness and a good inclination (Yetzer Hatov) that grows with moral and intellectual understanding and shapes the personality for good in proportion to the degree to which the heart adheres to God's law.
The primary expressions of conscience in the Old Testament are associated with experiences of guilt due to the failure to keep the law. From the beginning, law-breaking was viewed as exposing oneself to shame in the eyes of others (Gen. 3). David's heart was recorded to be "smote" (KJV), "troubled" (NASB), "conscience-stricken" (NIV) after he numbered the people (2 Sam. 24:10, cf. 1 Sam. 24:5) The Psalmist confesses "I know my transgressions, and my sin is ever before me" (Ps. 51:3) and prays "Create in me a clean heart, O God" (Ps. 51:10).
While conscience is primarily viewed as adherence to God's law, there are also biblical passages in which conscience appears as a "spiritual audacity" bold enough to argue and reason with God. Abraham questions whether the entire population of Sodom and Gomorrah deserves judgment (Gen. 18:20-33), Moses successfully intercedes for the Children of Israel when God declares his intention to destroy them (Ex. 32:9-14), and the Psalmist accuses God of sleeping while his people are given into the hands of their enemies like sheep to be eaten (Psalm 44).
Consciences free to challenge omnipotence and influence God himself will not be afraid of questioning the moral authority of their leaders and of challenging the injustices of any earthly potentate. Amos denounces societies that trample on the heads of the poor and deny justice to the oppressed (Amos 2:7), Micah objects to leaders who pronounce judgment for a bribe, priests who instruct for a price, and prophets who divine for money (Micah 3:11), and Ezekiel abhors the arrogance of cities like Sodom where there was abundant food and careless ease, but no help for the poor and needy (Ezekiel 16:49).