Hi. I am kind of in the same boat that you are in. I have fairly short cycles as well, about 24-25 days. I too have been using one of the sites online, mostly just to help me track my cycles each month etc. But it also tells when you would most likely ovulate. I don't put too much stock into it just because it's only an average. I asked the same question a few days ago here and from what I understand I can go ahead and start using my OPK as soon as AF stops. AF for me last six days and so after AF I just start using the OPK until I get a positive. So, I guess it's possible to O that soon after AF. I always tell my friends that my cycles seem so short that I am always either having AF or PMS! It's not a fun way to live sometimes!!!
Anyway, good luck to you. I am sure someone more knowledgeable about this subject could probably help you too. I am new here and still learning about all this myself.