93 izusu rodeo wont start

Just bought a 93 izusu rodeo 4x4 three days ago, the O2 dash light was on when I bought it and it ran great, the second day my wife drove it and it didn't want to run right, it was very sluggish when you stepped on the gas and when idleing if you stepped down on the gas fast it would die, now the third day it won't start at all, just cranks over and a loud backfire once, Please help me where shoud I start, fuel filter? O2 sensors?