I am trying to buy a one way train ticket from Stockholm to Oslo on August 02 and the search on RailEurope only brings up one travel time with a connection, is it too soon to try and book this? Also it mentions that a seat reservation is not included how do I include this?
Try booking directly from the Swedish Railways, www.sj.se instead of a middle man like rail europe. Right now I am seeing several trains for august 2nd, however note that there is track work in Stockholm, so you will probably need to depart from Stockholm Södra (South) instead of Stockholm Central.
Forgive me for sounding like a travel agent, but I would advise you to consider flying. It is (at best) a six + hour train ride compared to a 1 hour flight. It is certainly true that your train ride will begin and end in city central locations compared to the semi-remote locations of both airports - but - high speed trains will wisk you to and from central stations in short order. Flying may well be cheaper, too.
True, flying may same you an hour or two, but train tickets right now are as low as 235 SEK. That's cheaper than the express train to Stockholm airport alone.
235 kr is a great price Stockholm-Oslo. Flying will be a bit faster city centre to city centre but cost a lot more and in general be a more stressful journey. I would book the tickets.
And yes the airport trains in and out of each city are wonderfully efficient but are not cheap in and of themselves! Another price to consider along with the price of any flight. And you would have it on both ends!