The fastest way to the Berner Oberland is probably to fly from Venice to Zurich. I hate mid-trip flights, so I'd be exploring other options. Two scenic rail trips I'm aware of that connect Italy and Switzerland are the Bernina Express and the Centovalli line. Both would begin with a fast train from Venice to Milan.
For the Bernina Express you'd then need to head back east on a regional train to Tirano; that's the terminus of the Bernina Express, which is sometimes called Europe's most beautiful train ride. It ends in Chur (Switzerland).
For the Centovalli line you'd continue westward from Milan to the small town of Domodossola, which is west of Lake Maggiore. There you'd pick up the Centovalli train to Locarno (Switzerland). That's a sort of back-of-beyond trip.
Both of those trips would be scenic but long, significantly cutting into the time you have in the Berner Oberland (if that's your preferred Alpine destination). A faster route--again beginning with a fast train to Milan--would route you from Milan through Brig and Spiez; it looks like that would take at least 8-1/2 hours. I believe we have some Swiss-rail experts who can tell you how you might tweak that routing to avoid excessive time traveling through tunnels (at the expense of more time on the trains, naturally).
I believe any train route is going to involve multiple train changes, just so you know.
Swiss rail website: https://www.sbb.ch/en/home.html