I'm so sorry about your mc .. I had a mmc at 10 wks, the baby stopped growing at 7. It was heart breaking to say the least.
We wanted to TTC straight away without AF if I could time it right. We decided that if we could track O properly then we would give it a go. I used wondofo HCG tests everyday to check the HCG in my system, I also used OPKs at the same time, just to see the line variance. With any HCG in your system still your also going to be testing positive on the OPK. It's likely that is the case for you now.
I got positive HCG test until CD 27, counting the first day of red blood from the mc as day 1. As soon as the HCG tests faded to negative (they were REAL faint for about a week, but alas that still is positive), I got a truly positive OPK about 5 days later and Od the next day. . I also got pregnant. ...which lasted a total of 2 days... chemical.
If you read about the internet, as I'm sure you have.. you'll see lot of stories about women who go on to have healthy babies immediately after mc, and very few stories otherwise. I honestly think my chemical was due to a lining issue due to mc. I'm still not sure if I regret having tried straight away though- I felt like I was able to get back into control TTC straight away again in a situation where i felt helpless.
In conclusion, I say- test out HCG to neg, then MAYBE you MIGHT ovulate which you can track with OPKs. I hope you get your rainbow baby. <3