Not sure if I understand you correctly. You have made your own projects in some projection. After adding OSM via openlayers plugin your project CRS has automatically changed to EPSG:3857. If you remove the openlayers layer, you can switch back to any other CRS.
If your data is in a projected CRS, you can rightclick on the layer
-> Layer-CRS for project
If your data is lat/lon degrees, you might better choose a projected CRS, e.g. the UTM CRS for your part of the world.
There is no need to reproject the data, QGIS can display it in any CRS you want.