Should I use a singular or plural "one" here?

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?

Solution 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

Solution 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