Philips Android Smart TV's don't support the inbuilt native NOW App.
You could try using the inbuilt Chromecast with your Philips OLED TV, but i find it currently broken when watching NOW on Demand (only live NOW TV is working on my Philips Android Smart TV when casting from the NOW App on either my smartphone or tablet).
Chromecast on Philips Android should be already inbuilt on the television (so no need to download an App).
If you add the NOW App to either a smartphone or tablet from your mobile device Apps Store.
Then when you open the NOW App on the smartphone or tablet it should show the cast symbol towards the top of the NOW App on your phone or tablet.
If you then hit the cast button on your phone / tablet from within the NOW App it should cast to the Philips TV screen.
If you have a NOW Membership try a live NOW stream first which works for me on my Philips Android Smart TV.
Then maybe try some on Demand content which on my Philips TV doesn't currently play (sometimes hitting the play button a number of times on the NOW App on my smartphone will play eventually and sometimes no joy with the NOW on Demand stuff).
Forgot to mention if you are using the Philips OLED TV over wifi rather than Ethernet hardwired.
Then make sure your mobile casting device ie smartphone or tablet is on the same wifi Network has your Philips TV.
I've just bought the same type of TV (Philips 55OLED936) and have the same problem. Can connect to the built in Chromecast but can't stream on demand content. Bizarre issue, which I totally did not expect. It operates on Android so I assumed all apps would be available. Now TV no longer produce their stick, so what's the answer? I've just raised the issue with them but not heard back yet.
The answer is make sure you purchase or have a device that listed on the link below.
Personally I would get an Amazon fire stick for the time being until your device is supported.