Parc Guell and Catedral Familia Sagrada are quite far away from Hostal Opera and I would suggest taking the subway there. I guess it will take 30-45min by foot from Catedral to the hostel.
However do keep in mind that these 2 locations are not really located in the city centre. The main bustling area is still La Rambla

If you want to be more centrally located BETWEEN the Ramblas and Parc Guell (which IS pretty far north), you might want to stay closer to Placa Catalunya. You'd be closer to the Gaudi buildings (Eixample area) as well. Of course, that will put you farther away from the Ramblas and everything in the more central, southern part of the city. Hostals Goya, San Remo, Lazza, Que Tal, Eden, Barcelona Home Guest House and maybe Girona come to mind for a little bit farther north than Ramblas, but still close to metro and buses. However, I haven't stayed at any of these places (making my first trip to Barcelona in May; staying at Hostal Goya).