You can usually buy Robux on a phone by opening the official Roblox app, tapping the Robux icon, choosing a package, and then paying with the mobile payment method available on your device or region. Roblox Support says Robux and membership can be bought in the mobile app, and the purchase flow is to open the app, tap the membership or Robux icon, choose the amount, and confirm the pop-up.

On Android or iPhone

  1. Open the Roblox app and sign in.
  1. Tap the Robux icon in the app.
  1. Pick the Robux amount you want.
  1. Confirm the payment when the pop-up appears.

About OPay

Roblox’s official mobile payment options depend on your phone and region, and the sources I found specifically mention Apple Pay, Google Pay, app-store balances, carrier billing, and gift cards rather than OPay by name.

So if you want to use OPay, the most likely path is that OPay would need to work as a card or wallet funding source that your phone’s payment system accepts, not as a special Roblox checkout button.

Safer way to try it

  • Make sure you are inside the official Roblox app.
  • Check whether OPay is linked to a card or balance that your device can use for mobile checkout.
  • If the Roblox app shows only Google Play, Apple Pay, or another store method, use the one your phone offers.
  • Avoid third-party “cheap Robux” offers, because the official purchase flow is through Roblox’s own app or approved payment methods.

Example

If you are on Android and OPay is connected to a payment card, you would open Roblox, tap Robux, choose a package, then select the card or wallet method that appears during checkout.

If OPay does not appear there, that usually means Roblox in your region is not taking OPay directly.

Language note

I answered in English because your question was in English.