Should I use a singular or plural “one” here?

Question

Sometimes I get confused over how and when we should use plurals.

For example, should we say:

They are the only one who is capable of doing this

Or

They are the only one who are capable of doing this

When to use is/are? Why?

Answer 1

"they are ... who is.." jars to me.

I'd prefer:

When talking about only one person or entity/organisation:

He/she/it is the only one [who is] capable of doing this

When talking about more than one person or entities:

They are the only ones [who are] capable of doing this

Answer 2

When talking about only one person or entity/organisation:

They are the only one who is capable of doing this

When talking about more than one person or entities:

They are the only ones who are capable of doing this

Answer 3

The construction can be boiled down simply to:

They are the only capable of doing this.

capable of doing is an adjective phrase.

More obvious examples in the same direction would be:

Those maps are good, but the stamped ones are the only correct.

I do not know where they all are. The only I could find is on the table.

Similarly you can say "the best are/is", "the most are/is", "the nicest are/is"... as long as it is clear what subject you are referring back to. You can do this omission if the subject the word one or ones would refer to is close enough or to avoid repetition of one or ones.