Between the mechanics and the gameplay?
The mechanics are in the inner workings of the gameplay (button presses, "what you can do"). The gameplay is how they are implemented.
We are so quick to judge a game on false idols. We see two characters sparring and we immediately call it a fighter. We have to break things down to a conceptual level. If you look at Tobal 2 - which is based on the "Golden Combo" system - in terms of mechanics it has more in common with Dance Dance revolution than King of Fighters. You wait until 2 sprites overlap each other and press the button to continue the stimulus. However, the gameplay implementation is vastly different between the two. When you look at the interpretation of the mechanics between Tobal 2 and KoF you see them as similiar games (you're "punching" or "kicking"). You see the same thing in "racing games". One racer may have you press a button as fast as possible while another has you pressing directional buttons at key points to avoid obstacles (or even as drastic as fine tuning your engine and calculating wind resistance). One racer may be a drag race while another relies on the control of sharp turns.
That specific example above may have not been the best way to explain my view by bridging two unlike games together, but the best analogy I can think of (specifically to relate to Osman/Strider and mention games you'd be familiar with) where there are two similar games that play vastly different is the Pulstar -> Blazing Star comparisson.
Osman/Strider are not completely different in gameplay, but there are distinct features in each that set the gameplay appart. Let's not forget about Osman's Powerup feature compared to Strider's option/tiger feature. Osman's powerup feature where you have multiple images of yourself works perfectly in the sterile preplanned structures of the level. You can "set yourself up" in specific places because you know an enemy will be there. Strider's powerups, with their own seemingly random behaviors, works great for the hysteria and rich enemy interactions.
More games that look similar but play different:
Vampire Killer (MSX) : Castlevania (NES). Pretty much the same sprites, just different quality. Similar mechanics between each other (walk, crouch, jump, attack, secondary weapon), but the level structure makes them completely different animals. In Vampire Killer you are searching in a Metroid like level structure for a key to open a door to the next level. In Castlevania you are working in a linear structure. How the game is expressed is important. If Vampire Killer were in a linear structure without free roaming and branching paths then it would be useless to be searching for anything, because you'd be bound to stumble across it regardless. In Vampire Killer, the enemies appear at the same spots at exactly the same times (and the key is at the same place, too). So you'd only complete that game when you'd mastered it... when you memorized every step and found the shortest route. In Castlevania you may get lucky and complete it, and then fail on your next 5 attempts.
Strider/Osman's differences aren't drastic, but when you get down to the grit you can see what makes them unique.
A game that everyone in this thread may enjoy is "RunSaber" on the Super Nintendo. It plays like Strider and maintains Osman's atmosphere. It doesn't play like Osman, though.

Heh, in all seriousness, playing that game may help get my view across best.