Anybody been in the Osa and Arenal area during these months? Which one is the wettest?

I would go with July; you'll probably still get some rain, but not enough to ruin your plans. Although it's green season, we went to the Osa in July and didn't get a drop of rain. Not entirely predictable, but sometimes a little break from the rain comes in July. Even if you do get rain, it's rarely an all-day event.

I've been in early July and going back early next July. It was dry except a few rains at night. I agree with vgirl to choose July over August.

I agree, go with July. We went last year in the beginning of August, and while you just can't predict the weather - we had A LOT of rain. We were in Arenal, Tortuguero and Manual Antonio. In Arenal we had more than one day where it rained ALL day long. I've often hear people say it rains in the afternoon or at night and not ruin their plans, but all day rain, especially when travelling with kids - ruins your plans. For me, to spend so much on a vacation - if you have any choice at all, minimize the chances of rain!!

In Tortuguero we had no rain, and in MA, we had a few very heavy afternoon rains.

If we go back, which we're planning this summer, it will be late June or early July. It still will be rainy season, but with the kids and school - there's not much choice. And I'm not saying we didn't enjoy our trip - it still was a blast! But we were very soggy, much of the time.

We had a ton of rain in Tortuguero; I was worried it would never stop, but we got lucky and it did! We always pack books just in case. Hope you have good weather. For the areas you're interested in, I would probably skip September and October.