Due to the increasing popularity of the Oktoberfest, it has become difficult to gain entrance to tents during busy times. Generally, the busiest times are weekends and on Friday after about 2pm. At other times, there might be a short wait to get into a tent, but generally it is not too bad. In the mornings during the week, you will easily be able to go from one tent to the other.
Getting into a tent is far harder when the weather is bad than when the weather is good. When the sun is shining, a lot of people are happy to sit in the Beer Gardens surrounding the tents, which frees up the inside somewhat. Even on a weekend, it might be possible to walk into a tent at any point during the day if the weather is good. If it's bad, everything might be full by 9am, especially on a Saturday.
If you are intending on visiting the Oktoberfest on a Saturday or a Sunday, it is very important to get to the tent before opening especially if the weather doesn't look that great. Outside the bigger tents, a line will start forming at around 7:30am to 8am out the front of the venue.
Just because you are there at 8am doesn't necessarily mean you are going to get a seat. Firstly, the lines end up becoming very disorderly towards opening time. Furthermore, it is in no way guaranteed that the tent staff will open the front doors first - there are plenty of doors around the side of the tents too.
They will do this even if the majority of people, who have been waiting for over an hour, are at the front doors. Of course, as soon as someone sees that another door has been opened, there is a mad trampling rush to get there, and generally the first people to have arrived are in the worst position to get to these doors first.
The once you get into a tent, the biggest mistake you can then make is sitting down at a table that is reserved, as you'll no doubt be told to leave it as soon as the unreserved tables are all gone.
The general rule is that the middle section of the tents around the band podium are not reserved, at least on weekends. In fact, there is a local ruling that a certain percentage of the floor space must be unreserved on weekends and public holidays. In any case, any reserved table in this area should definitely have a sign on it saying "Reserviert" or "Reservierung" if it is.