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 this case you can look at the pKa values of their conjugate acids. Higher pKa of conjugate acid means a weaker acid hence a stronger base. The pKa of water is 15.7, while most alcohols have pKa of 16-18. 

I guess this question is asking for a general trend. Although alkoxide could be sterically hindered, for which an extreme case would be a tertiary alkoxide, but if the electrophile is not hindered at all (e.g. primary alkyl halide), then OR⁻ is still stronger than OH⁻.