Making function using 2x4 decoder and one or gate
The problem stated in the question looks to be impossible to solve. I will try to prove it here.
The function required is $$F=\bar{A}+ABC=\bar{A}+BC$$

First let's look at two different(and the only possible) configurations: 
1) The OR gate's output is connected to one of the sel inputs of the decoder. 
2) The OR gate's output is the function output. 

The first configuration assuming two of the function inputs to be connected to the OR inputs, and the third connected to the decoder input (and might be connected to OR as well):







simulate this circuit &ndash; Schematic created using CircuitLab

For the 2-input OR variant the outputs are given as follows:

x   y   z   Y0  Y1  Y2  Y3
0   0   0   1   0   0   0
0   0   1   0   0   1   0
0   1   0   0   0   1   0
0   1   1   0   0   1   0
1   0   0   0   1   0   0
1   0   1   0   0   0   1
1   1   0   0   0   0   1
1   1   1   0   0   0   1


As we can see neither of Y can be represented as SOP of 4. So this configuration won't work.
For the 3-input or variant:

x   y   z   Y0  Y1  Y2  Y3
0   0   0   1   0   0   0
0   0   1   0   0   1   0
0   1   0   0   0   1   0
0   1   1   0   0   1   0
1   0   0   0   0   0   1
1   0   1   0   0   0   1
1   1   0   0   0   0   1
1   1   1   0   0   0   1


Only \$Y_3\$ is SOP of four, but it's obvious that the function is \$Y_3=x\$, and no assignment of the original variables to \$X,Y,Z\$ will make it the required function. 
As for the second approach, while the OR is connected to the Decoder outputs, there is only two inputs on the decoder, then the third one has to be connected to the OR itself. But then the output will be always 1 when that input is 1. But, as can be seen from the truth table, there is at least one case for each input \$A,B,C\$ where it is 1, but the output is 0. Then this configuration doesn't work as well.