Melt multiple boolean columns in a single column in pandas

idxmax

df.idxmax(1).to_frame('OS')

        OS
0  Windows
1    Linux
2      Mac

---

np.select

pd.DataFrame(
    {'OS': np.select([*map(df.get, df)], [*df])},
    df.index
)

        OS
0  Windows
1    Linux
2      Mac

---

dot

df.dot(df.columns).to_frame('OS')

        OS
0  Windows
1    Linux
2      Mac

---

np.where

Assuming only one True per row

pd.DataFrame(
   {'OS': df.columns[np.where(df)[1]]},
    df.index
)

        OS
0  Windows
1    Linux
2      Mac