Question: How to determine the order of nucleophilicity for given chemical species?

Like I came across this question, to rearrange $\ce{RCOO-, OR-, OH-, H2O}$ (alkyl acetate, alkoxide, hydroxide, and water) in decreasing order of nucleophilicity and it ranked $\ce{OR-}$ first.

Shouldn't it be $\ce{OH-}$ since $\ce{OR-}$ would be more sterically crowded?

Answer: 
Within a group of nucleophiles with the same atom, the nucleophilicity decrease with **decreasing basicity of nucleophile**.

Decreasing basicity means the decreasing affinity of electron pair for a proton. The decreasing order of nucleophilicity is shown in the figure. 

[![enter image description here](https://i.stack.imgur.com/EwG8V.gif)](https://i.stack.imgur.com/EwG8V.gif)

But the relationship between nucleophilicity and basicity can be reversed by steric effect. Less basic but steric unhindered nucleophile therefore have higher nucleophilicity than strong basic but strically hindered nucleophile.
In addition to looking at pKa's, you must also consider the stabilizing effects of the surrounding groups. RO- has an alkyl group attached, allowing a greater amount of polarizability. This means Oxygen's loan pairs will be more readily available to react (ie they will not be held in as close and as tight) in RO- than in HO-. Consider Fluoride vs Iodide, the same concept applies. Fluorine has a smaller atomic radius and thus less polarizability (less area to distribute the charge over) than Iodide. Fluroide holds onto it's loan pairs much tighter than Iodide, and therefore the loan pairs are much less available to react.

Another great example of this, consider sulfides. They are generally better nucleophiles than similar compounds that contain oxygen instead of sulfur. This is due to the larger atomic radius of sulfur and therefore greater polarizability/charge distribution.

Also, negatively charged species are almost always more nucleophilic than neutral species (when considering the charged form vs the neutral form of the same molecule, such as water vs hydroxide or ammonia vs amide).