Using an oscillating saw for long cuts
The oscillating tool is NOT the tool for the job that you describe. Neither is the jig saw. The proper tool for long cuts like you describe is a circular saw with a straight edge guide clamped onto the work piece. Even better would be a table saw but that is clearly not applicable here. 

Get the right tool for a job. Why compromise the craftsmanship with shoddy crap cuts made by tools that are not made for the application. Invest in the right tools, do quality work and be proud of the result. If you cannot give it that then hire someone that can provide it for you. There is nothing worse than a kludged up mess.