Using section formula
Co-ordinates of A=(1+21×5+2×0,1+21×0+2×4)=(35,38)
Co-ordinates of B=(1+22×5+1×0,1+22×0+1×4)=(310,34)
Slope of OA=m1=35−038−0=58
Slope of OB=m2=310−034−0=52

We know that, tanθ=∣1+m1m2m1−m2∣
⇒tanθ=1+251656
⇒tanθ=4130