I’m building my version of a “janky” table that will be a 2x4 frame with a sheet board top. My intended use for the LR3 is cutting out (through cutting) mostly rectangular plywood parts for flat pack kits. I know Ryan suggests MDF, particle board, OSB and plywood (in that order) for materials. I plan to use a MDF spoil board on top of the table.
Given all the above, does anyone see issues with using OSB for the table top? Or would you suggest another material?
It really does not matter as long as it is flat. 
MDF and OSB will “swell” if it gets wet for a long time (like a spill that doesn’t get cleaned up), but you can seal MDF and it will help a lot. I’m not sure if it is possible to seal OSB in the same way. As long as you are somewhat careful to avoid exposure to liquids, you can probably skip the sealing.
MDF, OSB and Plywood will all sag if not supported properly and weight is applied unevenly over long periods of time, I’m not certain but I think OSB is the worst of the three for this.
If all you are doing is placing sheets of plywood on the table, the weight is fairly well distributed, so depending on the thickness, supports every 12" - 24" should probably be more than enough for any of the options. If you have any point loads (heavy item in the middle of a shelf span, for example, or even the weight of the table with a long unsupported section), that will likely cause bending or sagging of the span over time.
I personally really hate using OSB for anything other than what it was designed for, which is building cladding, roofing, and sub-flooring. In all of these applications the OSB is eventually covered on both sides. Exposed OSB is very rough, has lots of splinters, and looks ugly (IMO). It is also quite susceptible to water damage, and loses it’s structural integrity when it swells. (Same with MDF to a lesser extent, and plywood to much lesser extent).
Bottom line - OSB might be ok, as long as you support it properly and keep it relatively dry. Personally I would choose Plywood or MDF instead, but I have seen others here use OSB for table tops.
The OSB I use is made for subfloor, so it’s fairly water resistant. I then throw a few coats of poly on it once the top was built.
For material hold down method, I use screws, straight the material and into and through the spoil board.
I used to have two layers, bottom layer of OSB, and top layer (spoil board) of MDF. The screws would go into the OSB and bite and hold really good.
I currently have two layers of MDF, and it does not hold screws as well, easier to strip out. I kinda wish I had put a first layer of OSB, and then MDF on top.
I have not had this issue with my 2 layers of MDF. Could possibly be the screws you are using but hard to say. I find myself mostly using pocket screws as hold down. The nice flat flange on them works well. Most of the time I run them just next to the material and it holds down very well. I also use the course thread version.
Probably if I didn’t have anything to compare it to, I would not think anything about it. I have had the screws to strip out on rare occasions with the double MDF, which never happened with the OSB. I’m using course thread screws.