** Shares of nuclear startup firm Oklo OKLO.N down 14% at $18.65 after the bell
** OpenAI CEO Sam Altman is stepping down as the chairman OKLO, CNBC reports
** Oklo went public last May through a merger with Altman's blank-check firm
** Stock is up marginally so far this year as of last close
(Reporting by Savyata Mishra in Bengaluru)
((Savyata.Mishra@thomsonreuters.com))