Hi , I'm James DVM MRCVS and would like to help you with your question
Sorry to hear about Oscar
There could be a number of reasons for Oscar's lameness.
could be that he has some joint inflammation (at times it would be hard to feel if you are not pressing the right spot)
could be that he has pulled on one of his ligaments which is now painful .
it is possible that he is suffering from what is more commonly known as growing pains
I would say that , to the untrained eye , it might seem that one limb is the lame one when in truth its the contra-lateral limb (on the other side)
so it might be that you are feeling the wrong limb .
the fact that he wants to go and play is a encouraging towards a very good pragnosis.
Since it has been going on for a week , I would advise to be seen by your vet.
he could easily pinpoint the source of the lameness (might even deiced to perform an Xray) . He might decided to prescribe a short course of pain relief which will assist in the healing process as well.