opus est is a Latin idiom, and as an idiom it does not make immediate literal sense. The Noun opus means work. Cicero, for example, uses the expression opus quaerere to look for work. Ovid uses the following phrase to describe working in the fields; facere patrio rure opus to perform work in the ancestral countryside. Caesar, in his military writings, speaks of a place natura et opere munitus fortified by nature and labour.
opus can also describe the product of work. You would know the term magnum opus great work, defined by the Oxford Dictionary as a work of art, music, or literature that is regarded as the most important or best work that an artist, composer, or writer has produced. When Cicero speaks of the Silanionis opus he is referring to a statue created by Silanio. Elsewhere he says of a literary creation opus habeo in manibus I have the work in [my] hands.
And yet the expression opus est means there is need of. It is usually partnered with a Noun in the Ablative Case that expresses the thing that is needed. For example, opus est auxilio there is need of help. One theory is that the expression really means there is work [to be done] with something. And if that is true, it would make sense to put the with something in the Ablative Case as this is one of the Ablative Case’s favourite jobs (often referred to as an Ablative of Instrument). There is work [to be done] with help, therefore, there is need of help. QED.
The final piece of this idiom is a Noun or Pronoun in the Dative Case, which expresses who has the need. Ravena says mihi opus est auxilio, which hyper-literally means there is to me work [to be done] with help. Of course, no one translating would ever write it like that; they would write I need help. But if you are reading Latin as Latin, you won’t be thinking about English.
A few examples to finish.
pauper sum. mihi opus est pecunia. I am poor. There is need to me of money.
ieiuna sum. opus est mihi cibo. I am hungry. There is need to me of food.
sitio. opus est mihi aqua. I thirst. There is need to me of water.
solitarius sum. opus est mihi amicitia. I am lonely. I need friendship.
iter longum erit. opus est nobis equo. The journey will be long. We need a horse.
‘I need your help,’ the woman says. ‘My suitcase full of money was stolen from me. I should like you to find the thief – or thieves – for me.’