Actually, I’m not sure about many Catholics believing in OSAS, but I have seen many who believe that Hell doesn’t exist! And, well, if Hell doesn’t exist, that leaves 3 options: either all go to Heaven (which ends up being the default Christian belief for people who don’t believe in Hell), the souls of people not going to Heaven are destroyed (the JW view), or the atheist view that Heaven doesn’t exist either, and the earthly life is all we have. And if either option 1 or option 3 are the case, then there is no reason to reform one’s life.
Acknowledgment of the existence of Hell - and the real, true, possibility of ending up there due to one’s own choices in life - is necessary to understand the reason for a savior in the first place.
Honestly, the idea of OSAS is completely un-Biblical, and I have no idea how people who rely on sola scriptura could even come up with such an idea. Faith in Jesus saves, sure, but there are many, many passages in the NT alone that focus on the importance of one’s deeds done in faith and love. Even Jesus Himself stated explicitly that those who claim to follow Him yet neglect the physical needs of others (the “Least of my children”) will go to Hell (and though the spiritual needs are not explicitly stated in this passage, they are implied, as well). Paul chastises the Corinthians for tolerating (and even condoning) the incestuous relationship of a man with his mother-in-law, and actually tells the Corinthians to excommunicate the couple so that the couple can see the error of their ways and come back to Jesus. We all know James’s exhortation that faith without works is dead - that you can see his faith shine through his works. And in Revelation, St. John chastises the Christians in one of the churches as being “lukewarm”, mentioning that if they stay this way, Jesus will spit them out.
But that’s an aside. The important thing is that others must learn through our examples. It’s not that words aren’t necessary - they are - but people won’t listen with their ears until their hearts have been made ready. And the only way their hearts will be made ready is if we do all things lovingly ourselves and pray for their conversion. Seeing a loving, joyful example in life will make people want that, as well - and when they ask you how you can constantly be so loving and joyful, then you can start explaining your faith to them a little at a time, and maybe one day be able to lure them to Mass.