I am new to learning Latin, and I read on some random Latin site that "ob" means "on account of" or "because of". But, I thought that "propter" meant "on account of". Is there a difference in meaning that I don't understand? Are they just redundant?
The causal meaning of these two prepositions developed separately, so the history of their usage is a bit complex. But in Classical Latin and after, while some authors used the two interchangeably, ob became more and more limited to literary language, while propter was used in popular language.
Silvia Luraghi's paper "Prepositions in Cause expressions" traces the development of prepositions used to describe cause, like ob and propter. She first describes the difference in original meaning between the two – propter spatially means "nearby," so it comes to refer to cause through the idea that "a cause and its effect are contiguous." She gives this example:
abduce me hinc ab hac quantum potest, quam propter tantum damnum feci et flagiti
"take me hence away from her as soon as you can, on whose account I have incurred so much loss and guilt"
On the other hand, ob originally means "in front of." So if an observer is trying to look at a landmark, but there is an obstacle in between them, his view of the landmark is "exchanged" for the obstacle, and this is how the word is ultimately connected to cause. For example:
men piacularem oportet fieri ob stultitiam tuam, ut meum tergum tuae stultitiae subdas succidaneum?
“is it proper that I should be the atonement for your folly, so as for you to substitute my back as the scapegoat for your folly?”
(Pl. Ep. 139-140).
So we see that the two words have different origins. But what about usage?
History of usage
In Early Latin, Luraghi finds that ob (109 occurrences) is more frequent than propter (49 occurrences), but in examination of Plautus, she finds "the onset of a development that took ob to be limited to idioms."
This trend continues, she argues, in Classical Latin. She summarizes:
The distribution of ob and propter is of special interest. In certain authors, the two prepositions seem to be equivalent. However, interchangeability of ob and propter is a comparatively late phenomenon, and may have never been a feature of the spoken language.
She cites Löfsted, who finds that in post-classical times propter was the popular word and ob the literary word. But she argues that evidence for this distinction already exists in Classical Latin. She notes that many classical authors show a distinct preference for one word or the other; for example, Tacitus rarely used propter, perhaps indicating that he felt ob was more suitable for literary works.
Others used the two fairly evenly – Cicero has ob 744 times and propter 885 times, though 60% of his usages of ob are in conjunction with rem or causam.
In Late Latin, she points to the absence of ob in non-literary texts:
Non-literary texts offer evidence for the disappearance of ob, which only occurs 3 times in the Mulomedicina Chironis, and does not occur at all in the Peregrinatio Aetheriae.
For today's Latin student, perhaps it's simplest to call the two words synonyms – Bennett's Latin Grammar (§141) translates them identically. Keep in mind that ob often goes with rem and causam (Gildersleeve and Lodge (§416)), and may sound more "literary." But if Cicero used both, you certainly can too!