This from the Wizards of the Coast website regarding OGL and software.
Q: So I could make a game?
A: Sure. Remember though, you cannot use any Product Identity with the
OGL or claim compatibility with anything. So you can't say your game
is a d20 System game or uses D&D rules or call it Elminster's
However further down the page they say you can't make an interactive game?! Huh?
Key components of games are goals, rules, challenge, and
Copyright does not protect the idea for a game, its name or title, or
the method or methods for playing it. Nor does copyright protect any
idea, system, method, device, or trademark material involved in
developing, merchandising, or playing a game. Once a game has been
made public, nothing in the copyright law prevents others from
developing another game based on similar principles. Copyright
protects only the particular manner of an author’s expression in
literary, artistic, or musical form.
Material prepared in connection with a game may be subject to
copyright if it contains a sufficient amount of literary or pictorial
expression. For example, the text matter describing the rules of the
game or the pictorial matter appearing on the gameboard or container
may be registrable.
It appears that WoTC are fudging the legality a bit.
You can make a game called Dungeons and Dragons (title) but it can't look like the dungeons and dragons title which is trademarked, or contain any other distinct likenesses of trademarked WotC elements, ie certain monsters, settings, characters.
WOTC might have patented the Dungeons and Dragons game system, but it appears that they have only patented the magic the gathering system, and all collectible trading card game systems somehow, not really sure how that works...
It would also appear that they can't patent DnD rules, if they haven't already, as it appears you can't patent something after a year has passed since you released it publicly.
Most info I read though, says the guy with most lawyers wins. I'm gonna patent that system so every time someone with more lawyers wins, they need to pay me royalties.
As for WotC being overly litigious, the only examples I read were when some guy released early pics of their new MTG cards, one guy made a porno version of DnD using the d20 and WotC said we don't want that associated with d20 so he removed the d20 logo and released it under OGL, which differs from d20.
Summary, you can make a dnd-like game using OGL even if it doesn't specifically allow games(if you ban an interactive game you effectively ban all games as interactivity is a key component of a game) but you would need to change the names of most identifiably DnD stuff. But the WotC website seems to confuse OGL with d20 trademarked system, so I also did initially. This may be intentional to dissuade people from making a computer game based on OGL.
You can make a game of any type based on OGL, but not on d20. Everything in these system reference documents(SRD) is OGL:
Down the bottom they even say they can't copyright a game, and half-way down they say you can make any type of content using OGL. Main rule is you must identify clearly somewhere in your game distribution what content is OGL and you can't use the d20 content but the SRDs from the link above cover nearly every rule you need anyway.
BTW I'm making a game from the SRD in Unity. First step is a character generator which is mostly done.