Osbourne makes a quick return to Scottish football after leaving the Dons at the end of last season while 18-year-old Simon Colina Dominguez arrives from the Nou Camp.
The midfielder joined the Barca academy, La Masia, when he was 12 years old and last season captained the Juvenil B under-19 side.
Manager Alan Archibald told the club's official website: "I think we are signing two quality players here.
"Isaac is the experienced SPL player who I have been very keen to bring and I have only had Simon here for a couple of weeks earlier in pre-season, but you can see that he has got plenty to offer.
"It will be interesting to see how quickly he settles into the Scottish game but from an ability point of view, he certainly has that in abundance and it will be great to watch him progress."
Former Coventry midfielder Osbourne was offered a new deal by Aberdeen but rejected it in order to be closer to his family in Birmingham.
The 26-year-old, who made 53 appearances in two seasons at Pittodrie, was on trial at Blackpool earlier this month.
He said: "I did enjoy my football in the SPL so to be able to continue playing in the top league is ideal for me.
"I have had a few days here at Firhill and I see it is a very young squad but they are determined to do well this year and I hope that I can help make this team a success in the coming season."