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.
OR¯ is NOT RO¯ !!!
Clearly, R¯ is an unknown species and nothing else can be said about it.
OR¯ would be an unusual structure, given oxygen's electronegativity compared to carbon's. In the case of RO¯, the R "group" would have to be very large and bulky to dramatically sterically affect (hide) the O¯ from other molecules. So for simple organic compounds, the effect will be quite small. As already said pKa of the conjugate acid would be an excellent guide to nucleophilicity (in the relevant solvent).