If you're approved online for a credit card then it will take 7 to 10 days to get the approved credit card in the mail so you won't be able to use a credit card the same day you get approved online.
But you can use the credit card the same day you get the approved credit card in the mail once you activate the credit card.
The credit card will need to be activated by you either online or on the phone before it can be used.