Oops -- forgot to take the OG

Not once, but twice, on two different batches of cranberry melomel/cyser (different recipes, one adds the fruit in primary, the other has more apple juice and adds the cranberries in secondary).

Anyway, without having the original gravity, am I going to have trouble figuring out when this is done? Or can I just let it ferment until it clears, racking once or twice along the way?

Just let it ride. Melomels & cysers tend to finish <1.005 regardless of the OG, because honey is highly fermentable and any sugars from fruit will ferment 100%.

The only problem I can see for you is that you won't know the ABV... Which can be a good thing if it's really tasty :rockin:. As david_42 said, as long and you check the FG, you will know when it's done.

Man, I've done that so many times! If it's really important, create a brew-day spreadsheet. Beersmith is PERFECT for this. A day or 2 before, get your starter going (if you will use one, some people also prepare/measure water at this juncture) and plan your brew day on paper. The 20 minutes you spend planning could save you hours of damage control on brew day. It makes for MUCH smoother brewing, with fewer mistakes.

Gotmead.com has a calculator for meads, if you still have your recipe, plug it into the calculator and you'll be able to get an idea of where you started. Don't forget to take an FG though! No calculators for that here's the link

mike