Another option would be to have an IUD placed. If Paragard (the copper iud) is placed within 5 days of unprotected sex it is 99% effective in preventing pregnancy. Then you also wouldn't have to worry about birth control until you're ready to get pg again. It's more expensive than Plan B obviously.
Married to my other half
Mama to Madeline (07-10-07) and Finley (10-17-11)
I've used both-- Plan B after a broken condom, and the copper IUD as regular birth control. I like the IUD a lot-- but only if you're going to wait a few years for #2, not really worth it just for a few months.
Plan B made me cramp and bleed a lot, but no long lasting side effects.
Just because the OPK is negative today doesn't mean it mightn't be positive tomorrow or the next day, and Plan B is more effective if you take it sooner. I'd go for the Plan B or the IUD like the PP mentioned. Plan B will probably make the rest of your cycle kind of wonky, so be prepared for your chart to be garbagey.