Pandas Use Value if Not Null, Else Use Value From Next Column

In [8]: df
Out[8]:
  COL1 COL2
0    A  NaN
1  NaN    B
2    A    B

In [9]: df["COL3"] = df["COL1"].fillna(df["COL2"])

In [10]: df
Out[10]:
  COL1 COL2 COL3
0    A  NaN    A
1  NaN    B    B
2    A    B    A