First of all - make a mental note that studies at universities and polytechnics/UAS's are not quite interchangeable, they are two separate strands of Finnish higher education. A handy albeit a bit simplified way of perceiving the difference would be academic - vocational higher education.
Then, keeping in mind that this distinction is also valid in open university studies, check the info on these sites:
Open Universities: http://www.avoinyliopisto.fi/en-GB/
Open Polytechnics/UAS: http://www.amk.fi/avoin/en/index.html
(You may also find info on the institutions' own web sites, but it's best to read those two first.)
Then - remember the following.
1) Open university studies do not as such lead to any degree, and are not considered "full-time" studies.
2) You can not get a student residence permit on the basis of Open University/UAS studies . In case your residence is already considered permanent, this is no problem. But if you live abroad, open university is not really a very realistic option TBH.
3) The open university studies are not free of charge. How much and how you need to pay depends on the institution, some may charge a fixed annual fee, some may charge separately for each study module, etc.
4) Open university/UAS studies are not an automatic way of gaining a degree study placement. Most higher education institutions have an "open university channel" of admission, which basically means that once you've completed a certain number of credits you are eligible to apply on the strength of your open university studies. But that's not a quarantee of a study placement! You may need to pass an interview, for example, and in many cases they also look at your open university grades. Plus, the annual open university admission quota for a particular degree programme can be very, very limited.
5) Even if you are a Finnish citizen, you can not get KELA student aid for open university studies. The reason - see point 1.
So - open university/UAS is not an easy way into university, it is an alternative way. It may be an option for some, but plan carefully and make sure you have the correct info before embarking on this option. This can be best obtained by contacting the institutions, not through forums like this
