Neapolitan song written in 1898 by Giovanni Capurro (lyrics) and Eduardo di Capua (music). Not a traditional as often stated. The authors sold the rights for 25 Lire (!!) to a publisher. Ever since a show case tune for overweight Italian tenors and overweight Rock’n Roll Kings („Now or Never“). Not to mention overweight ukulele players like me.
At the 1920 Summer Olympics in Antwerp, "’O Sole Mio" was played when the music to the Italian national anthem could not be found (says Wikipedia).
Not too difficult. Comprising intro, verse [ A ] and chorus [ B ]. During the long notes it takes some filler notes or light strums or just let ring. Toughest part is the Bb minor (drama !!) with the jump up the fretboard.