It is hard to offer advice about how to fit something into a box without knowing the size of the box, but this is essentially what you've asked people to do. That is, because your example isn't complete, we can only guess your choice of font, default font size, paper size, margins and so on. A better answer would require more complete information.
In what follows, I assume that you are using the article
class with default settings. This means US letter paper (216 x 279 mm) in portrait orientation with 10pt Computer Modern serif, sans and typewriter, and default margins, skips and so on.
I suggest setting the tree out 'directory-style'. Depending on the potential width of the nodes' contents, you might either typeset the whole tree this way or switch to this style at an appropriate point.
If the nodes' contents may actually be significantly wider, you might set it like this:

showframe
is used to mark the text block, margins etc. Obviously, you shouldn't load this in your real document, but the lines are useful here.
If the nodes' contents is really short, as in the example, I think it looks better to switch after the first children:

Here forked edge
is used to get a similar style for the edges between the root and its children.
As of Forest 2, these styles are simple to implement using the edges
library, as the following code illustrates.
\documentclass{article}
\usepackage[edges]{forest}
\usepackage{showframe}
\begin{document}
\begin{forest}
for tree={folder, grow'=0}
[Tests
[Case 1
[OGL1]
[OGL2]
[OGL3]
[OGL4]
]
[Case 2
[OGL1]
[OGL2]
[OGL3]
[OGL4]
]
[Case 3
[OGL1]
[OGL2]
[OGL3]
[OGL4]
]
[Case 4
[OGL1]
[OGL2]
[OGL3]
[OGL4]
]
]
\end{forest}
\begin{forest}
where level>=1{if level=1{forked edge}{}, folder, grow'=0}{}
[Tests
[Case 1
[OGL1]
[OGL2]
[OGL3]
[OGL4]
]
[Case 2
[OGL1]
[OGL2]
[OGL3]
[OGL4]
]
[Case 3
[OGL1]
[OGL2]
[OGL3]
[OGL4]
]
[Case 4
[OGL1]
[OGL2]
[OGL3]
[OGL4]
]
]
\end{forest}
\end{document}