98 z havin a cat problem look an see what i mean

ok g/f just baught a brand new cat direct fit up to the manifold from summit the brand of the cat is catcoput it on an the new exhaust an baught a new universal o2 sensors an it didn't fit.Now do i have't to get a oem stock o2 sensors or do u think they welded the wrong bunt in so that my o2 sensor wouldn't fit?