We just bought a house and it turns out one of the windows seems to be painted shut. It's a reasonably recent double paned wooden double hung window (say around 15 years old). The house was painted for the showing and while most of it is surprisingly good the painters got a little sloppy on this one. Could someone recommend how to get it unstuck.
I don't think I'd use a knife. It's very easy for the blade tip to grab the wood grain and climb out of the groove. Now you have a slice where you don't want a slice.
I don't think I'd use an oscillating tool. It seems much too coarse for this job. You'll end up with a fairly ratty edge on both the sash and the jamb or sill, especially if clearances were tight to begin with or you don't have the blade at the right angle with respect to the joint.
I do think I'd take a putty knife and a hammer and gently tap the knife into the crease, just through the paint thickness, working along the entire width. Keep the knife flat against the sash to prevent damage. The reason this works well is because it cuts and wedges at the same time, resulting in minimal damage before the paint pops loose.
I'd then use a foam sanding block or similar to ease the edge that remains on the sash and the jamb and/or sill.
Another tip: [edit: removed suggestion to use putty knife after box knife. Putty knife alone is better]
If the window doesn't have a handle, don't use the top rail to push violently upward. You can cause damage to the window by pulling up too hard on the middle of the top rail. Instead, screw a couple of metal handles diagonally across the bottom of the sash, with screws in both the vertical and bottom rail on both sides. Use the handles to manipulate the window until it moves freely.
Score as deeply as you can with a utility knife. The problem is here, they are not flat. Your standard box cutters will nut cut a straight parallel line because of their housing. So don't go to deep or the angle may cut into some of the wood.
There are many things you can use but I often use a reciprocating saw blade - just the blade. These are usually thin enough to get into the crack your created when you scored. I have seen guys use drywall knife and other types of knives (long steak knives - you use what you have).
I've had luck with a "multi-tool" like this Dremel. Just run the blade carefully along the painted over bits between the window and the jamb. Some sanding and touch up may be required. It's a pretty safe tool to use. Like Solar Mike said, slipping with a knife could cause an injury which could cost way more than a Dremel tool!