What Is the Primary Factor That Causes an OTM Option’s Delta to Approach 1.0 or -1.0?

The primary factor is a significant movement in the underlying asset's price, causing the OTM option to become deeply In-the-Money (ITM). As the option moves deep ITM, its Delta approaches 1.0 for a call or -1.0 for a put.

This signifies that the option price will move nearly dollar-for-dollar with the underlying asset's price change. This transition is known as the option's "moneyness" changing, and it is crucial for option valuation.