Notation for repetitive nucleic acids
dN denotes deoxyribonucleoside (where N can be A, T, G or C). If there is no d prefix, then it denotes a ribonucleoside.

Regarding the difference between poly and oligo:

The former refers to having a large number of monomeric units whereas the latter means having a few monomeric units. There is no strict definition of the boundary between poly and oligo. Poly-A tails are typically about 70-80nt long in yeast (Eckmann et al. 2010). So it logically follows that anything shorter than that should be called an oligo-A. Chemically synthesized stretches of DNA are often referred to as oligos even though nowadays one can synthesize more than 80nt long DNA/RNA. Moreover, a stretch of As in some mammalian mRNAs smaller than the usual length of ~150-250 is still referred to as a short poly-A tail instead of an oligo-A tail (Jalkanen et al. 2014; there are many more examples).

Additionally, even if the cutoff is, let's say, 100 units, it doesn't apply to proteins. A protein with 100 amino acids is still considered a polypeptide and not an oligopeptide. For peptides, the cutoff is generally around ~30. 

So, I would conclude that the usage of these prefixes is quite contextual.