Really confused

took my van inspected. it through code PO1040. We changed to 02 sensor that was bad
(i had the dummy light on i knew it was the o2 sensor). Then i drove my van about 15
min. and it stalled at a red light (otherwise was running great) i restarted the van and it
stalled at the next red light (about 2-3 later) and stalled once more in my drive way. I
asked the mechanic about it and he said it was the computer readjusting after the 02
sensor and replaced. I drove the car the next day it didn't stall. Now day 3 i drove it
about 25 min down the road and it stalled out. I was able to restart it and dropped off my
kids and went to my campus ( another 15 min. away) and it stalled out it the parking lot
(got the van parked after 10 min of effort) went to my class came back out and it started
right up but i went straight home where it stalled in the drive way and now wont start at
all (it tries but just wont)