How we analyze or assess the Old Testament in terms of credibility and reliability depends on how we define those terms. If we equate credibility and reliability with factual accuracy, the Old Testament fails the test: we have, in many instances, no way to objectively verify the literal historicity of what occurs, and, in addition, much of what occurs, such as the parting of the Red Sea, the faithful surviving being thrown into a blazing furnace, a woman being turned to a pillar of salt, or a man living for three days in the belly of whale, are so unlikely as to be incredible.
However, if the Old Testament is understood as a set of stories gathered over a very long period of time by a people trying to communicate their conviction of being in a covenant relationship with God, the stories suddenly start to cohere and have a credibility and reliability through the repeated messages they communicate.
Throughout the Old Testament, for example, we are given illustration after illustration of individuals who are kept safe because they are faithful to Yahweh (God), as well as many stories of those who suffer or die because they turn away from that relationship. For example, the story of Lot's wife being turned into a pillar of salt because she disobeys God by looking back at the sinful city she is leaving behind might make no literal sense, but it makes metaphoric sense: it conveys the more profound truth that we will eat ourselves out with grief and tears (salt) if we keep looking back in longing to the life we left behind once God calls us away from it. Similarly, Jonah is punished by God for disobeying him, but finds peace once he obeys what seems a senseless calling: this coheres with the message repeated in the Old Testament that asks to think outside of the box of mere logic and, instead, accept that sometimes God calls us to tasks that may not make rational sense. Likewise, even fantastic stories of salvation, such as the parting of the Red Sea, illustrate the coherent theme that God is powerful and protects the faithful.