"The Last Leaf" by O. Henry, what can be a good thesis and conclusion?For example we learn in the story that through power of friendship, Hope made...

Yes, friendship and sacrifice for others is basically the theme of this work, but O'Henry's understated manner of telling the story emphasizes the fact that sacrifice can be hidden in love.  The story gives the best conclusion for sacrificial love:

" . . . look out the window, dear, at the last ivy leaf on the wall. Didn't you wonder why it never fluttered or moved when the wind blew? Ah, darling, it's Behrman's masterpiece—he painted it there the night that the last leaf fell.”

Greater love has no man than he who lays down his life for another. That is your conclusion if you think about it.  Think about how unselfish Johnsy's friends are. Look at her reaction to the leaf and how she realizes suddenly that SHE has been selfish. A masterpiece, that leaf, was all she needed and all the old man could give.

Good luck with your work.