The offensive line (or "OL" for short) is made up of five players split across 3 positions: Tackles (left and right, abbreviated by "LT" and "RT" respectively), Guards (also left and right, abbreviated by "LG" and "RG"), and the Center (abbreviated by "C"). The linemen set up with the tackles on the ends of the formation, the guards next to them, and the center in the middle.
In DR and the NFL, offensive linemen have two responsibilities, pass blocking (preventing anyone on the defense from getting to the quarterback) and run blocking (pushing defenders out of the way to make room for a running back). Both of these jobs use specific attributes.
Pass blocking uses Strength, Pass Block, and Footwork as the most important attributes, immediately followed by Read Opposition and Athleticism. First Step also plays a small role.
Run blocking uses primarily Strength, Run Block, First Step, and Motor, with Footwork playing a small role. An Offensive lineman is typically between 290 and 330 lbs. Strength and size are tied together, so an especially heavy player will be stronger than their Strength/Size attribute would suggest, and vice versa.
Position specific notes:
Center: A center must have, above all else, at least a decent Read Opp, or his team will have terrible trouble with blitzers. A good snapping skill can help reduce fumbles. Keep in mind that centers are typically lighter than an average lineman, with 290-300 being about the norm. Also, a player does not have to be listed as a center to play there. if you have a guard or a tackle with good Read Opp, Snapping, and Athleticism (to reduce the penalty for playing out of position), they should be just fine.
Guard: Typically players value Run Blocking ability a little more at guard than at center or tackle. Also, like centers, other offensive linemen can play guard if you so choose.
Tackle: A LT should be the best pass blocker on your line, period, while your RT should be a capable pass blocker but also have some run blocking skills. Unlike guard and center, you cannot play any player other than a natural tackle at this position.