I logged into the Oyster website at tfl.org and it still says I have
only 90p, not 10.90. Do I need to wait longer for the money to register
or do I need to do something more to validate it or something?

The Oyster Card system is asynchronous. The only place to get your most
up to date balance is from the card itself.

When you check your balance online you get the last balance that has
been communicated from the card via a barrier or validator back to the
main system.