The Southeast slope is perhaps the shortest route to the summit of Old Man Mountain, however, most of the route is cross-country hiking. The route is considerably shorter if you have a high clearance 4WD vehicle. The route begins as a descent, into the valley to Fordyce Creek. Once you reach the creek, you must cross it, then climb directly above up the southeast slope. If you park at Fordyce summit, the route is about 6.5 miles roundtrip. If you do have a 4WD, you have the option of making the hike 4 miles, with a 2300 feet of total climbing.
To get to the southeast slopes of Old Man Mountain, you should exit if I-80 at Cisco Grove and take the road north over the Yuba River. Make a left when the road reaches Donner Summit Rd, just after the river. Turn left and then turn right in a few 100 feet onto Forest Road 85. Take this dirt road up the valley. You will see Signal Peak up to your left in some parts. Take the road past the Woodchuck Flat campground and up to the Fordyce Summit. Here, hikers with passenger cars will have to park. If you have 4WD, you can continue driving another 1.25 miles. Continue on the road that has the sign that points to Fordyce Lake. Take a right at the next major fork, and then the road will begin to descend. Hike/drive down this road for approximately 1.25 miles until you can see the easily identifiable gap in the ridge that is just to the left of the road. You can park where the road comes to the gap, and here is where the cross-country part of the hike begins.
Hike through the gap and you can clearly see the entire route before you. Choose your best way to descend to Fordyce Creek. This 500 foot descent is over loose rock and some thick brush. Once you hit the creek, find a way to cross (there was a nice crossing log when we were there). Be careful, because the creek seems more like a river in terms of water flow. After you have crossed, hike briefly through the lush forest until you come to the noticeable Fordyce Creek OHV rock-crawling trail. Cross the road and begin to climb directly above you. You can pick your route how you like going up the southeast slope. Climb 1700 feet above the creek on the large slabs and talus, and you will eventually come to a false summit. From here, you can easily see the summit. Head to the northeast (right) along the ridge and this will bring you to the base of the final summit talus slope. Just ascend this and you will be at the summit. Be sure to walk to the other side of the summit to check out the northwest face. Re-trace your steps back to your car.
Good hiking boots for all of those pointy rocks!