What is the difference between an ordinary Catholic who tries to practice his faith, although he may not always succeed, and someone in Opus Dei?

There is no essential difference. A person in Opus Dei is an ordinary Catholic who has received a specific call within his or her Christian vocation to form part of a spiritual family and to try to be a better Catholic and help the Church carry out its mission. This call does not distinguish someone in Opus Dei from others, because it does not entail that their life be transformed into an ecclesial sign of following Christ, as is proper to the religious vocation. Secularity is an essential characteristic of all the faithful in Opus Dei, including those who live celibacy as part of their vocation. Obviously, if by an ordinary Catholic one understands a Catholic whose life is not especially committed to our Lord and to the Church, then the faithful of Opus Dei strive with all their strength not to be recognized as such. For they realize they are called to remind others that what should be ordinary for Christ’s disciples is to follow him unconditionally, with a lifestyle that could be surprising to someone who does not share their Christian perspective.