This is nice, but if you look at the trendline there, it is pretty apparent that it has more to do with the particular methodology of the poll (Daily Kos polls) and its particular ideosyncracies than with any real late-breaking trend towards O'Rourke.
Yes, it is clear across polls that O'Rourke has gained ground and is doing comparatively well, but in this same poll, you can see they had O'Rourke and Cruz tied 1 year ago (which strains credulity, as that was well before the campaign had even started), and also had O'Rourke up slightly earlier in this year.
So it is good news, but doesn't mean he is likely to win.
At least, it gives hope, thus giving the necessary optimism for the Dems to win against Cruz in Texas.