o = a: 123, b: 394, c: 321

you can do:

o<[a b]> # => [123, 394]

which is great, if you have keys = <[a b]>, this should work too:

o[...keys] # => [123, 394]

(note that o[...<[a b]>] does work)

same thing for array should work as well:

[1,2,3][...indexes]
where indexes = [1,2]