Not quite opposing. Lets take for example an Sn1 reaction (leaving group has left and nucleophile is attracted to carbocation). Since we're working with a carbocation here (+ charge) you want something with a negative charge to satisfy it (more negative the better). How do you get a more negative charge you ask -> electronegative atom. In this case, you are right that you need to follow periodic table trends to see that Oxygen is more negative than Sulfur. Therefore, ethanol is a better nucleophile.
but I thought I read that high electro neg also ment that the atom wouldn't want to share the e- with the carbocation since they are more tightly in electro neg atoms....or am I completely wrong
Erm, only if you're talking about that single atom. For example, an oxygen is a good nucleophile, but a carbon next to a carbonyl is a decent electrophile (and this has nothing to do with the electronegativity of the carbon itself, only its position on the molecule)
Several good points have been raised here. I'll try to address them in turn. Here's the bottom line if you don't want to read further. Thiols are more nucleophilic than alcohols, all else being equal.
Here's the more nuanced explanation. Nucleophilicity and electronegativity are
not equivalent concepts. Electronegativity is one thing to consider when looking at nucleophilicity but it does not by itself determine nucleophilicity. Since nucleophilicity is a kinetic phenomenon, you also have to consider steric environment, solvent caging, etc. when determining how nucleophilic something is. For instance, a potent nucleophile placed in a protic solvent will have its nucleophilicity attenuated due to solvation effects. So when you're comparing nucleophilicity, you need to keep all aspects of a molecule equivalent. That is, you couldn't fairly compare the nucleophilicity of tert-butyl-hydroxide with ethanethiol.
In this case, we can assume that these are in similar solvents and we know that they are similar sterically. So the only difference here is the atom - oxygen versus sulfur. Now, it doesn't make sense to talk about nucleophilicity of an atom. There's no such thing. No atom is ever by itself in a reaction (almost). Nucleophilicity of a single atom is meaningless. But we should look at electronegativity differences between those atoms. Sulfur is less electronegative than oxygen and therefore will hold onto its electrons less tightly. The direct corollary of this fact is that it will also more readily
donate its electrons away. Thus, based on this fact and this fact alone, thiols will be more nucleophilic than alcohols and thiolates will be more nucleophilic than alkoxides. This is why nature chose to use acetyl-CoA as the quintessential natural nucleophile.
For a more thorough treatment, see here:
Thiols And Thioethers — Master Organic Chemistry
