Which of the following words does NOT contain a prefix (उपसर्ग)?

Let's break down each word into its prefix and base word:
* प्रभाव: Contains the prefix प्र- ($\text{प्र}+\text{भाव}$).
* अपवाद: Contains the prefix अप- ($\text{अप}+\text{वाद}$).
* पराजय: Contains the prefix परा- ($\text{परा}+\text{जय}$).
* ओढ़ना: This is a root verb containing the suffix '-ना' ($\text{ओढ़}+\text{ना}$). It does not contain any prefix.

Therefore, ओढ़ना is the word without a prefix.