It's "Oga's," not Olga's, FWIW.
If you arrive at Oga's and get "seated" (or rather, stood at a table) quickly, you can enjoy a quick drink before heading over to Fantasmic! If what you want is a more leisurely experience to soak in the ambiance at Oga's, you might have to decide on the fly whether you prefer to stick around there, or head over to Fantasmic! (which will be a good 10-15 minute walk depending on your speed, since it's at the extreme opposite end of the park from Galaxy's Edge). If you arrive just before the show starts, you'll also be sacrificing the better seats (or if it's really crowded, the chance for a seat at all), since many guests arrive 60-90 minutes early to score the best ones.
My personal feeling is that if you can get into Oga's at another time, do that, so you don't have to rush around. Then again, my family's experience at Oga's was uniformly horrible (rude server, filthy table, 15-minute wait to order, 30-minute wait for our drinks after orders were placed, and when they were finally dropped off after we begged another server to help us because ours had disappeared, 3 of our 4 orders were incorrect!!!) , so I'm a little salty towards the place.