Are notations intended to look nice? I was just wondering: when mathematicians come up with new concepts that requieres new notations, do they only look at how practical it is, or does asthetic also play a role? For example, I have seen both $\mathcal{O}_X$ and just $O_X$ as a notation for the structure sheaf of a ringed space here, and while I believe that very often "canonical reasons" are available to choose one notation over the other, are there examples of notations where the creator just thought it looked nice, and are there even examples where notations are more made to look nice than to be practical?
This raised the question wheter mathemacians care about beauty, which I would certainly answer with yes, but also what beauty is: rather an elegant argument internal in mathematic logic, or also the way it looks written down on paper?
 A: Mathematical notation tends to be strongly reflective of what it's trying to say and 'aesthetics' don't come into play very often.  It's also worth noting that notation is a consensus-affair; very few mathematicians get to unilaterally pick a notation and have it stick.
For example: the equals sign $=$ was introduced in $1557$ by Robert Recorde who is noted as saying that two parallel lines were the most equal thing he could think of; prior to that people tended to write words to indicate that two things were considered equal.  The symbol speeds up the writing of mathematics, but there's nothing particularly interesting about it.  A second example would be that Stefan Banach talked about what we now call Banach spaces as "spaces of type B".  He may have been hinting at wanting them named after himself, but it was later mathematicians that granted him that honour.
Very often things are named by taking the first letter of a word describing them: $\mathbb R$ denotes the Real numbers but this is most likely good fortune for English speakers since the German word for the real numbers is reelle Zahlen which also begins with an R.  The integers are denoted by $\mathbb Z$ because the German word is just Zahlen.  The integral sign is famously just a long 's' standing for 'sum', and the summation sign $\Sigma$ is again just standing for 'sum'.  In the comments g.kov notes that the square root symbol also arose in the same way (possibly from the letter r or perhaps an arabic letter).  Slightly harder to decode, but still named in the same way, Hausdorff introduced $F_\sigma$ and $G_\delta$ sets where G is the first letter of Geöffnete (open) and F the first letter of Fermé, while the Greek letters indicate countable summation $\sigma$ and intersection ($\delta$ for durchsnitt) respectively.
Because notation is consensus-based it can take time to settle down and during that period there may be competing notions in use.  In this case people using one or the other have to be more precise and it can spark long-running arguments.  For example, whether $\mathbb N$ includes $0$ or not (many write ${\mathbb N}_0$ to indicate that $0$ is being included) seems to be under debate at the moment.
And notation can be specialised in different ways for different areas without (usually) causing too much confusion: $\pi$ to a geometer can be expected to denote a famous irrational number, whereas to an analyst it may be expected to be a projection operator.
But by and large how pretty it looks isn't a big concern.  Being easily memorable for describing the set/operation/concept at hand is much more important.
A: In the past, before computers, printing was mechanical,  and, due to the human effort and mechanical limitations inherent in typesetting, notation was far more limited. Math books didn’t sell a lot of copies even back then, so spending a lot of time typesetting them was not cost-effective.
To typeset $\mathbb R,$ you had to actually have a cache of physical $\mathbb R$ characters, at least one for every occurrence on a page.
This meant that a lot of niceties, like fonts to indicate types, were less likely to be used. So $\mathcal O_X$ was a nicety unlikely to be found in early texts. In those times, there were a few special symbols. To distinguish types, they might distinguish capital and lower case Roman letters, and capital and lower case Greek letters.
We still have “hidden notation/conventions.” For example, we tend to treat variables $n,m$ as integers, and $x,y$ as real. This is particularly noticeable in the limit notation:
$$\lim_{n\to\infty}\quad\text{versus} \quad\lim_{x\to\infty}$$
Most mathematicians know these  mean two different things, even though there is no “logical” reason for the change of letter to change the meaning. In theory, the notation should indicate the domain of the limit, but in reality, we usually don’t need it.
Computer typesetting makes it much easier to invent clearer notations. Not just fonts, but superscripts and subscripts are much easier. Something like:
$$\underbrace{1+1+\cdots +1}_{n\text{ times}}$$
was nearly impossible with physical typesetting.
As for aesthetics, that depends on what you mean. Clarity has a certain aesthetic nature, and some of us find simplicity and clarity an aesthetic of its own.
But notation is meant to communicate clearly, and usually briefly. Most notation is only “beautiful” in the mathematicians’ eyes, the way that proofs and definitions can be beautiful, by making ideas seem more transparent.
That said, there is a risk of too much notation. The tendency to use logic symbols $\forall,\exists,\lnot, \land,$ and $\lor$ outside the setting of logic can be unclear unless the statements are very brief. They are appropriate on blackboards during lectures, because the lecturer is trying to be brief and speaks their meaning, but in text, English is preferred.
How much notation we use is an aesthetic choice, and requires conscious efforts at clarity. An ugly page is hard to read.
An example of notation I hate, personally, is $\vec v.$ It works for small numbers of vectors, but with a lot of vectors, the arrows seem too noisy. If I need to distinguish vectors from scalars, I much prefer to use bold face, $\mathbf v.$

It is worth noting, one reason we use Greek letters so much is that most educated people in Europe were taught Latin and Greek. This also meant that printers, particularly academic printers, tended to have a set of Greek letter.
