Normally I don't do peoples' research for them, but I was curious and at the same time wanted to explain ActiveX a bit. ActiveX is just a .dll file, an application extension. It uses a well-defined interface and the C calling convention. You can build them yourself from straight C if you know what you're doing, so any IDE has "support" for ActiveX.
OCX files are typically for Visual Basic users, or early IE versions, and are usually just controls (making your own button or combobox or textbox or whatever else). Or collections of controls of course, and you could write your own flash player as a single "control" if you want. It actually started IIRC with .vbx controls which were pretty much intended only for Visual Basic, then .ocx controls any OLE application could use, then we have ActiveX which are .dll files.
So when you said it has to build dll and exe files, any IDE that runs on Windows will do that. When you said it has to have ActiveX OCX support, that doesn't make a lot of sense. ActiveX is backwards compatible, so you can make a .dll instead of .ocx file. So still .dll files would qualify. Windows-only compilers might make .ocx files, but you'd have to find an old version (earlier than maybe year 2000).
What I think you're looking for in an IDE is something like Visual Studio where you can write a class or object or whatever and the compiler will write the ActiveX interface for you. That makes it windows-centric immediately, since no other platform has ActiveX. And most of the decent IDEs are cross-platform, so it does not sound like you will find what you want. Visual Studio 6 is the best match and it's not free.
Win9x support excludes a lot of programs. Visual Studio 5 and 6 would let you make .ocx files, and support Win9x, but it's not free. Modern Visual Studio versions have a free version but no IDE, and no OCX support. ActiveX yes, but .dll files not .ocx files. Visual Studio does not compile ReactOS source code. That is being worked on, but the code still uses gcc/mingw syntax in places and MSVC does not understand it, or there are linking problems, or the headers are misarranged so you have to include different files.
Here's a good starting point for you to find what you need:
http://en.wikipedia.org/wiki/Comparison ... .2FC.2B.2B
More info on making your own ActiveX interface if you want to go that route. I highly recommend this since you should learn how these work if you're going to use them:
http://stackoverflow.com/questions/1670 ... codeblocks
If you find something that works let us know.
EDIT: I assumed you meant that the output runs on ReactOS and Windows. Your criteria wasn't clear on whether the IDE should actually run on ReactOS. Since it's Alpha as of this post, there's no guarantee of course.