You need barriers with Oyster readers to use them. The rail companies don't have them.
What this means is that people who have their season ticket on an Oyster Card will have to be let through the gate by the attendant. Most people who have an onward journey from London Bridge by tube or bus will be in this category. I'd guess that covers a pretty high percentage of those entering during peak hours. It will be interesting to see if they check them all on hand readers or just wave everyone through.