I think the most likely situation in which Olga could have found herself heir would have been if Alexei had died at Spala. If Nicholas was going to change the succession at all, that would have been the moment. She was then not quite 17, and could then have continued her education and general development as the future monarch.
However, we have a further 'what if'. If Olga became heir in the autumn of 1912, when would she have succeeded? Nicholas was 44 in 1912. Romaov men were not long lived, but he could have expected to live another 15 years or so before dying from natural causes. If Alexei had died in 1912, Rasputin's role would have been much diminished (and, I think, Alexandra's as well). Of course, the war would still have happened, but would the REvolutions have happened when and as they did? Of course, we don't know.
Would Olga have made a good ruler? Of course, Alexander III only succeeded because of the death of his elder brother, and in Britain both George V and George VI were second sons who made excellent rulers. However, I don't think Olga was 'autocrat material'. If she and her advisers had managed a transition to constitutional monarchy, she could have made an excellent monarch, but Olga as autocrat would not have worked. Perhaps the tragedy of Nicholas was that he tried to be an autocrat while being totally ill-equipped for it, and failing to see the need for managed change.