Having a fear is an anxiety thing, a delusion is a perceptual thing. The difference would be being afraid someone was going to burn the house down versus believing someone was actually now buring the house down. Your actions would play out differently too. To keep future fears at bay, you'd check to make sure your house was locked and with OCD you might be afraid you hadn't checked well enough so you'd do it more often than necessary. There's a fear you wouldn't be able to deal with any problem that might come up. If you were delusional, you might start screaming or grab a "weapon" or run out another door to get away, etc. You wouldn't have the "space" to fear because the fear would be upon you in perceived reality and you'd be dealing with it instead.

It's three-way; fearing a fire, perceiving/thinking there is a fire, there actually being a real fire. It seems almost silly to think of someone having to say, "I was sure there was a fire!" as an actual fire wouldn't need any "conviction"/convincing of others since one would feel the heat and get burnt, etc. That's how I help myself with dreams that "seem so real" is check it against all five senses; maybe the monster was roaring and chasing me but did I smell his fetid smell/breath? Nope, didn't smell anything, just a dream. Seeing and feeling the spiders crawling on one is not enough, can I smash them? No? Then they aren't really there.

I have had trouble occasionally with checking irons too often (and/or literally leaving them on and having to come home from work and check them (or not because I was disgusted with my behavior and decided to take a chance only to get home later that day to find I'd left it on so could have burnt the apartment building down)) and other behavior like that and I think if I have problems in the future I'm going to write myself very large notes and put them on the door to be seen at a distance; "You already checked this one, it's locked!" :-) I think eventually I'd get unhappy with all the notes and would go back to trusting myself. With the irons, I would do "worst case scenario" (okay, the apartment catches on fire, the fire department is a block away and uses your building to practice on at least once a month so they'd arrive quickly. Your stuff would all be lost. Hard but I could live with it) and decide not to go back and check. Leaving the iron on once or twice made me anxious enough so I checked well before I left from then on and remembered checking because of my literal, heightened sensitivity from having left it on because I couldn't remember whether or not I'd checked. I changed the "rules" of the game from whether or not I'd turned the iron off to whether or not I remembered and that made a lot of difference in how I felt. I made remembering concrete and whether one remembers or not is not usually as anxiety provoking as whether or not one is going to burn an apartment building down :-)