Green’s Dictionary of Slang

certainly! yes!

1896		E. Turner Little Larrikin 12: ‘Hurry up now and be a good kid.’ ‘Right-O!’ said Lol cheerfully [OED].
1897		Barrier Miner (Broken Hill, NSW) 27 May3/1: ‘You’ve learnt some Australian slang, too!’ ‘Righto,’ laughed Chirgwin.
1898		Windsor & Richmond Gaz. (NSW) 10 Dec. 7/4: ‘Put the —?!!! brakes on, you —,(),**, fool!’ Shunter: ‘Rightoh!’.
1900		Marvel XIV:343 June 15: Right ho!
1902		A.N. Lyons Hookey 91: An’ I says, ‘Righto, boy!’.
1907		Gem 16 Mar. 8: ‘Right-oh!’ said Logs.
1910		H.G. Wells Hist. of Mr Polly (1946) 25: Right O! Don’t bag all the crust, O’ Man.
1911		Gem 23 Sept. 9: ‘Hold on a tick while I get the light!’ said Manners. ‘Right-oh!’.
1914		Joyce ‘Grace’ Dubliners (1956) 160: ‘We can meet at half-seven,’ said Mr M’Coy. ‘Righto!’ said Mr Cunningham promptly.
1914		Sport (Adelaide) 10 Jan. 10/3: ‘Six pots, Les.’ ‘Right O’.
1917		A. Brazil Madcap of the School 76: ‘Right-o, old sport!’ returned Morvyth.
1919		‘Bartimeus’ ‘The Wooing of Mouldy Jakes’ in Awfully Big Adventure 24: ‘Right-ho!’ he said gruffly.
1921		‘Dornford Yates’ Berry and Co 10: ‘Right-oh,’ said my sister, rising. ‘Is ten-minutes-to early enough?’.
1921		D.L. Sayers Unpleasantness at the Bellona Club (1977) 89: Righty-ho! I’ll have the bus ready for you at eleven.
1923		Wodehouse Inimitable Jeeves 47: ‘Oh, right-o,’ I said.
1924		C.J. Dennis ‘A Holy War’ in Rose of Spadgers 47: ‘Righto,’ I sez (an’ turns me back on doubt) / ‘I’m with yeh, parson. I go down to-night / To Spadgers’.
1928		‘Sapper’ Female of the Species (1961) 130: Right ho! mate.
1936		D. Stivens Tramp and Other Stories 192: ‘Come for a walk,’ she suggested presently. ‘Righto,’ I said.
1937		D.L. Sayers Busman’s Honeymoon (1974) 69: Righty-ho!
1939		K. Tennant Foveaux 35: ‘I’ll ask you to withdraw the remark.’ ‘Well, right-o.’.
1941		K. Tennant Battlers 57: Right-o, you be ready.
1942		A. Christie Body in the Library (1959) 49: So I said righty ho.
1942		Whizzbang Comics 63: Right-ho, Podger. It’s us for the North Sea again.
1949		H. Miller Sexus (1969) 95: We’d start a new life together, righto!
1951		(con. 1941) E. Lambert Twenty Thousand Thieves 95: ‘Righteo!’ exclaimed Temple, and the next minute swerved the truck.
1953		L. Uris Battle Cry (1964) 278: Righto, old bean.
1953		K. Williams Diaries 30 Jan. 88: I left him with R. in Cranbourn Street, he was exclaiming ‘Rightie Ho! Lyons. Cups of tea. Rightie Ho’.
1954		Wodehouse Jeeves and the Feudal Spirit 27: Right ho. I only mean it must have been pretty gratifying.
1955		D. Niland Shiralee 123: Righto, mate.
1957		C. Himes Imabelle 124: ‘Right-o,’ the cop said, hurrying off.
1957		Galton & Simpson ‘The Old School Reunion’ Hancock’s Half-Hour [radio script] Rightey-ho, mater, I’m ready.
1960		Wodehouse Jeeves in the Offing 21: She must have drooped her eyes and come through with a murmured ‘Right-ho’.
1963		K. Waterhouse Jubb (1966) 47: ‘Just half a pint, please,’ I shouted. ‘Oh, righty-ho’.
1965		C. Himes Rage in Harlem (1969) 126: [as 1957].
1965		W. Dick Bunch of Ratbags 113: Right-oh, Cheryl, I will.
1971		Wodehouse Much Obliged, Jeeves 103: ‘Tell Bastable [...] But nobody else.’ ‘Right ho.’.
1977		K. Gilbert Living Black 35: Righto, if you see a kid or anybody doin’ anything out of place, you belt him.
1980		A. Payne ‘All Mod Cons’ Minder [TV script] 49: Righto. Tell Arthur I’ll settle tomorrow.
1981		A. Weller Day of the Dog 8: ‘Right-oh,’ agrees Floyd.
1981		T. Wilkinson Down and Out 43: ‘Right-ho,’ he said.
1986		R.G. Barrett Godson 90: ‘Righto mate,’ he smiled. ‘Next stop Kempsey’.
1992		R. Barnard A Fatal Attachment (1993) 166: Right-ho. Go ahead.
1996		Penguin Bk of More Aus. Jokes 388: Righto, youse lot, quieten down.
1999		N. Cohn Yes We have No 193: I thought to myself, righty-ho.
2006		D. Mitchell Black Swan Green 366: ‘I should be off.’ ‘Rightio.’.
2006		Age (Melbourne) 6 Jan. 🌐 Righto let’s go. Rip into them.
2015		N. Cummins Tales of the Honey Badger [ebook] Never one to get into a barney, I say ‘righto’ and keep going.
2017		T. Spicer Good Girl Stripped Bare 115: ‘Rightio [...] There’s something else we want you to do’.