Another help is to use T-pins from the local "Office Despot" to hold the first rail in the line you desire. First spike that rail many ties apart to establish the direction, give it the eyeball test, tweak the installed spikes as needed then come back and spike every 4th tie or whatever you desire as your personal standard.
Roller or 3-poiont gages can be a big help to hold the second rail in gage ahead of the area you are spiking. I don't know anyone who sells O-scale rollers commercially but I made some for use by friends on the A&O track gang. One difficulty with roller gages is that they should be custom made with grooves to match the width of the rail head you use, and also gaged to match the standard practices of the layout owner when he employs other gages such as the NMRA. I've observed that some hand-layers run the NMRA as a hard minimum go/no go gage and spike the track close to the tight side of tolerances. Others might try to run the middle of the tolerance. Both are technically correct but two gages that conform to different uses can produce frustration, just as "a man with two watches can never tell the time!"
All the best.
Bob
PS: If I can figure out how to include images from my Smugmug account just as I used to do very easily, I'll add some photos to what I described. So far, for me, this new forum has been one step forward and three steps backward...