Are you part of the POPS club on campus? That's a great place to start, because then you get to know Dr. Shulman, who heads up admissions.
1) The earlier the better if you have had time to raise your GPA enough and are happy with your OAT. Schedule a time NOW to meet with Dr. Shulman and go over your current situation and get his guidance on how to best set yourself up for admission. That's a big part of his job (to meet with prospective students). This summer would be perfect to do that.
2) If you aren't doing summer school, then take your OAT in the middle or end of the summer after your 3rd year. By then you should have completed most or all of the pre-req's. You definitely want to finish Bio 1 and 2, Physics 1 and 2, Chem 1 and 2, and Orgo 1. If you are taking it sometime in undergrad, an anatomy/physiology class would also really help. I took my OAT before taking biochem and microbiology and I had no problem with the bio section. Use the summer time to study hard for the exam so that you can do well the first time you take it. Otherwise you have to wait 3 months to take it again. If you have finished all those classes already and want to take it at the end of this summer, your scores are good for 2 years (and you would be applying for admission about 1 year out from having taken the test).
3) Dr. Shulman will tell you that the most competitive candidates for UHCO will have "above a 700". What that means is you combine your OAT score with your GPA. So with your current GPA, you would get 325 points and then need a 375 on the OAT (or a 380 because it's only in increments of 10). If you had a 3.4, then you would get 340 points for that and need a 360 on your OAT, etc. That's not what all applicants have, but that's what he says is very competitive and those students will have a really solid chance of getting in plus getting their $1000 scholarship (the only one they offer).
I highly recommend meeting with him as soon as possible, because he can give you some good guidance on classes he recommends for the upper division classes. I think he suggested molecular bio, cell bio, and any kind of neuro anatomy classes available when I met with him. I applied this cycle and was accepted to UHCO but will be going to Berkeley. UHCO's clinical program probably can't be rivaled though!
Also, one of the doctors that works for UHCO allows prospective students to shadow at the various clinics he works at around town. I would recommend shadowing him, but his name completely escapes me at the moment. I know Dr. Shulman will know who it is though.
Good luck!