Green’s Dictionary of Slang

r. adv.

[abbr.]

right, correct.

[US]Sun (NY) 16 Nov. 2/6: Thgere is little need of cautioning you as I know you are ‘R’ O.K.
[UK]A.N. Lyons Arthur’s 12: ‘Goin’ up to London?’ says she. ‘R!’ says I.