Oil Viscosity Grade
 

if my car says use 5W-30 oil, is it ok to use OW-30 ?

if you believe what they say on BITOG, then yes, cold 0w and 5w are both thicker than hot 30 weight, the oil only gets thinner as it heats up. could there be an advantage to extra thickness at low pressures and loads when the engine is still cold? most seem to think no.
one advantage is if it gets really cold (like well below freezing) then the 0w may start a tiny bit easier than the 5w. if it never gets cold the 5w may be a little cheaper.

so its safe to use 0W-30

I would think it would be safe, and certainly using a slightly incorrect grade oil is far better than using oil that is beyond its life or, even worse, letting it get too low. The engine won't die immediately or anything, though it's possible that the overall engine life might be slightly reduced...or it might not.

Still, why not follow what Honda recommends? They've done the engineering and research for the engine.

I am using the correct grade for my fit, this is for my aunts lexus. Because i have an extra 0W-30, that i will put in her trucks engine, her lexus is due for oil change.
What happened is i was using royal purple, but people said its not good, so i bought another brand, then decided to buy another brand i wanted to return the first one i ordered but the seller is giving me a hard time i decided to use that oil for my aunts truck

I find that the oil weight spec'd by the manufacturer for the mid to hooter climates normally matches my driving style and usage.

For instance my BMW uses 5w-30 for normal driving and 10w-40 or 50 in hotter weather. The variable valve timing reacts a little slower and faster based on starting viscosity and temperature. I use the Lubromoly Oil in a 5w-40 and the engine is noticeably quieter than trying to run the 5w-30. (Even using the same brand...) And since it takes 7.5 quarts and your supposed to run it up to 12K miles I use a top end full synthetic. Redline, Amsoil, Lubromoly, or the dealer synthetic which happens to be fairly economical and is good oil its castor I think in a BMW bottle.

There's no harm on using 0w-30. Both oil will have almost the same viscosity at operating temp.