Cold Sores and Oral Herpes are the same thing.  They are a virus that once you have, you can never get rid of.  Oral herpes can be contracted at a young age from a kiss from a parent, sharing a glass at school  etc.  It is more contagious during an outbreak and remains contagious until the sores have completly healed.  It is therefore possible that you have kissed someone without realising that they had the end of a cold sore.  It is however very rare to contract the oral herpes virus by kissing someone who just has the virus but does not have a sore.

If you had caught it, you would probably have had an oral outbreak by now - so try not to worry about it and make sure you do not kiss anyone who has a cold sore!