Simplest & cheapest approach would be (as others have suggested) using a junior hacksaw to cut through the olive almost parallel to the pipe until you can break it off with a screwdriver in the slot you've cut. The only thing I'd add to that us use a *good* junior hacksaw and *good* blades. Been there, done that .... having used el cheapo saws and blades and now using an Irwin
I've removed a lot of olives using this technique and it works, the only problem typically being actually getting access to work the hacksaw in a confined space and the remaining pipe tail possibly being too mangled by an over-tightened olive and/or valve twisted out of line with the pipe. A dollop of Boss White will make up for a lot of imperfection in the pipe, but if it's too bad you may have to cut back the pipe and solder on a new bit. For this you'll have to have the pipework draind right down, and if the pipes are buried in concrete that may mean sucking the water out with a wet vac.