The original commandment, "Do not bear false witness against your neighbor" (Exodus 20:16 and Deuteronomy 5:20) focuses on a specific aspect of lying, and is directly applicable to the testimony a person might give in court. Other Scripture passages focus on the general (and all-encompassing) principle: "Do not lie," or something similar (examples - Leviticus 19:11; Colossians 3:9).
By implication, what we are supposed to do - to be truthful - is also a part of this command. Truthfulness, integrity, honesty and other similar traits are to be characteristics of our lives.
As for oath-taking, there are times when a person may rightfully "swear to tell the truth, the whole truth, and nothing but the truth" - such as in a court of law. To use what Jesus says, in the "Sermon on the Mount," to oppose such oath-taking is to tear his words out of context. He was teaching us that our character should be such that there would never be a reason to question the truthfulness of what we say - oath or no oath. We shouldn't have to take frivolous oaths every time we say something, in order to get people to believe us!
There are times when we have a moral obligation to withhold truth from someone - such as when a person is not an authority over us, and has no legitimate right to the information. But withholding information is not the same as lying about it. (Some of the articles below explore this issue further.)
A group of Scripture passages which show us the seriousness of making (and keeping) the promises and vows we make. We should not make promises or vows lightly!
This is a general article that looks at various issues related to the question. Numerous Scripture passages are included, which demonstrate that it is not right to lie.
This is some e-mail correspondence I had with a person who asked about Rahab's lie - whether or not it was a sin. Many people try to use James' comments about Rahab to "defend" lying. Yet a careful examination of James 2:25 shows that such is not the case.
This is some e-mail correspondence I had with a person who claimed that God did tell Samuel to lie. We never reached an agreement, but the "dialogue" is instructive, in that it brings out some of the issues that come up when this issue is discussed. It also shows some of the reasons (or assumptions - whether good or bad) that cause people reach their conclusions.
Truthfulness as a lifestyle...
Simply let your 'Yes' be 'Yes,' and your 'No,' 'No'; anything beyond this comes from the evil one. (Matthew 5:37)