A layout or a subcell of it can be saved to either GDS2 or OASIS. To save a layout, choose "Save As" from the "File" menu. To save just a cell, select the cell in the cell tree (it does not need to be the currently shown one) and select "Save Current Cell As" from the context menu (right mouse button) of the cell tree.
A file dialog will pop up to select the file name to which to write the cell or layout. After a file has been selected, an option dialog will be shown to specify further options. In this dialog, it is possible to constrain saving to a subset of layers, i.e. just visible ones. Also, the database unit can be changed or the layout can be scaled by a given factor.
For OASIS, a compression level can be specified. At a level of 0, no particular attempt is made to compress shapes. At higher levels, shapes are classified and array compression is tried. The higher the level, the more attempts are made to compress shapes into arrays. In particular for flat layouts, compression of shapes requires some memory and slows down OASIS writing considerably.