It's still possible he'll sign with St Mirren. Clubs that size operate very differently, most players are on one or two year contracts, so every Summer there's a lot of players leaving, renewing and a lot of players becoming available. Once the season ends it takes time for things to be decided because the budgets are so tight and you don't know who's staying and who's leaving yet.
He made a decent impression, although has a few weaknesses in his game, one of the better keepers the club has had recently. Next season the target is a promotion challenge though, so it might turn out Jack Ross (manager) wants a more experienced keeper, despite the positive impression O'Brien made. O'Brien himself has made some positive noises about wanting to stay, but i imagine he'd also likely wait a while and see if any offers appear down south, the wages for the same kind of level are significantly higher.
St Mirren are also in the process of selling a young player to Barnsley, the fee received for the that will have a significant impact on the transfer/wage budget, so there won't be many decisions about the playing squad before that's done.