Should I soak old-fashioned oats?
You don't even need to cook them, let alone soak them.
They form the bulk of most mueslis, to be eaten raw.

However, if you're going to be making porridge...
The basic recipe has as many variations as there are people who cook it, but take any or all of


Water
Milk
Salt
Sugar [or honey or anything sweet]
Oats


Use approx 300ml liquid to 50g oats per person as a start point. Bring to the boil, simmer for 5 - 7 minutes depending on your desired consistency.  Very "old fashioned" jumbo oats will still boil down to nearly mush if you leave them long enough, though most people want that bit of bite left to them.

Some make it thin, some make it thick. Some make it thick but then add cold milk to their dish. Some make it smooth, some leave it less-cooked still with big bits in. Some cook with the sugar in, some only add it at the end.
Some leave out the salt, though personally I think it's essential.

There is no hard &amp; fast rule; make it how you like.

From comments...
The bigger &amp; less refined the oat, the longer it will take to cook right down to a 'paste' - but often the entire idea of using big rustic oats is not to overcook them, but leave some bite.
I wouldn't dream of pre-soaking them, there's absolutely no need. If you want them cooked to paste, cook longer, or change back to a more refined brand. Some of the outer part will probably not cook right down anyway with very 'rustic' oats.

The Guardian - How to cook perfect porridge goes on at length about various methods, but comes down to pretty much what I've already written above, with no pre-soak [not one single mention of rinsing, btw, just don't do that].