you can, but please make sure the journey is within London zone 1-9 for example, you can take a train from London Bridge to Greenwich by using your O Card, but you cannot take a train from London King's Cross to Cambridge by using your O Card. And please note: it may be more expensive to use the train than tube. For example, you may be charged more than GBP 3 if you use a train from London Bridge to Greenwich, but it may cost less than GBP 3 if you take a tube for the same journey.
for example, you can take a train from London Bridge to Greenwich by using your O Card, but you cannot take a train from London King's Cross to Cambridge by using your O Card.
And please note: it may be more expensive to use the train than tube. For example, you may be charged more than GBP 3 if you use a train from London Bridge to Greenwich, but it may cost less than GBP 3 if you take a tube for the same journey.