Both Oracle and SQL server are suited for OLTP. Other factors need to be considered to decide for one or the other, or to choose the specific edition (of any of them) that best suits your needs.
Either one will work just fine for an OLTP database. You need to evaluate the features that both have in the various editions (Express, Standard, Enterprise, etc) to see which one has the features that you want.
And then you’ll need to look at the price of the packages to see which one fits in your budget.
I did some of the pricing analysis on my blog.