Search Parameters OR

You would probably want to look into ESGST

I don't think it is possible but why not making two bookmarks? One for level and one for entries.

https://www.steamgifts.com/giveaways/search?level_min=1&OR&entry_max=200
Is that possible?

Adjust that to OR=True, or OR=1, and it should definitely be possible [for cg to implement]. I forget if you can default set HTML form variables [ie, preset the OR variable to 1 and be able to automatically switch it to 0 if it's in the command line without any set parameters], but there's no readily apparent reason that it shouldn't work if you clearly set the paremter [eg, to 1], since or functions are inherent to database queries to begin with (in the same manner that and functions are).

Well, I'm in favor of phrasing it as "Blend=True", though, but I guess that's an inconsequential point. :P
As my preference on that detail indicates, all you're really doing is blending two different searches into one, so it definitely seems like something a script could handle. If you're willing to poke around in the code, userscripts should always have all their data visible user-end, so you could just dig through ESGST [given the indication by ChessPie that it already has the function] and try stripping out the relevant section(s) (for use in a new script of your own), if you'd prefer to not utilize the full ESGST script.