In the nineteenth century, there were sections of the Republican Party who called themselves "Radical Republicans” and “Liberal Republicans." This would not have been an oxymoron during this time period. Radical Republicans were those that were not willing to reach a compromise with the South during Reconstruction. They were radicals because they were committed to their principles. On the other hand, Liberal Republicans were those that wanted to return to a purer exercise of liberalism.
Nowadays, the terms are likely to be considered an oxymoron because we normally associate the term "liberal" with the Democratic Party, which is the other major party in the United States.