After charges have been pressed against a criminal and the person had been summoned in court, an omnibus hearing commonly takes place to determine several factors important for the upcoming lawsuit. The omnibus hearing is used when the defendant pleads not guilty, in order to determine whether or not the case can be held up in court at all - that is, whether there is sufficient evidence against the defendant, and if their testimony is sufficiently convincing and plausible to clear them of their charges (in cases where evidence/witness reports are lacking).
There's a difference between an omnibus hearing and a contested omnibus hearing - in the first case, the process is a more generalized one, asking the defendant some default questions that need to be covered, as well as determining suitable dates for appearing in court and clearing out any other important details which will be referenced in the lawsuit.
On the other hand, a contested omnibus hearing has the purpose of a pre-trial defense attempt by the defendant - if they feel the court has insufficient evidence or other grounds on which to press charges, the defendant may challenge the court to prove that the case is eligible for a lawsuit. This can be used to save some time and legal fees in cases where the defendant is confident of their eventual acquittal. Defendants have the right to not attend an omnibus hearing, in which case they're precluded from pleading not guilty until the case has started, after which it proceeds as normal.