Some are, some aren't. it depends on the razor. You should have a look through the review section at a few and see what people have to say about individual razors.
For example the Merkur open comb is rather mild, however the old style Gillette is aggressive .The R41 and slant are not open combs and they are some of the most highly regarded aggressive razors, so it is all rather subjective.
As stated above; it really depends on the razor and the person and their beard and skin type. You would have to decide once you try them. The Merkur OCs are quite mild, but if you put one shim in them, it really improves the shaves, in my opinion.
The vintage OCs are usually a bit more aggressive. You may want to start with a user grade Old Type to see what you think as you can pick one up for under $20 usually, or a NEW Long Comb, etc.. The more aggressive vintage ones are the OC Aristocrats, but they tend to be much more expensive. There are a few modern OCs that are nice; the FaTip is the cheapest of the lot and quite aggressive. If you are thinking of the Muhle R41, save that for later when you have improved your technique unless you are brave and want to go at it; very, very aggressive/effective razor. The teeth on most of the OCs are fairly strong, in my opinion. The OCs are very good and effective shavers, especially, if you have coarse hair and sensitive skin. Good luck.